cannot build `microhttpd` module on CentOS 8

``` CC (gcc) [M memcached.so] memcached.o CC (gcc) [M memcached.so] mcd_var.o LD (gcc) [M memcached.so] memcached.so CC (gcc) [M microhttpd.so] microhttpd_mod.o microhttpd_mod.c:418:24: error: return type is an incomplete type static enum MHD_Result ksr_microhttpd_request(void *cls, ^~~~~~~~~~~~~~~~~~~~~~ microhttpd_mod.c: In function 'ksr_microhttpd_request': microhttpd_mod.c:434:10: warning: 'return' with a value, in function returning void return MHD_YES; ^~~~~~~ microhttpd_mod.c:418:24: note: declared here static enum MHD_Result ksr_microhttpd_request(void *cls, ^~~~~~~~~~~~~~~~~~~~~~ microhttpd_mod.c:459:10: warning: 'return' with a value, in function returning void return MHD_NO; ^~~~~~ microhttpd_mod.c:418:24: note: declared here static enum MHD_Result ksr_microhttpd_request(void *cls, ^~~~~~~~~~~~~~~~~~~~~~ microhttpd_mod.c:474:12: warning: 'return' with a value, in function returning void return MHD_NO; ^~~~~~ microhttpd_mod.c:418:24: note: declared here static enum MHD_Result ksr_microhttpd_request(void *cls, ^~~~~~~~~~~~~~~~~~~~~~ microhttpd_mod.c:481:10: warning: 'return' with a value, in function returning void return MHD_NO; ^~~~~~ microhttpd_mod.c:418:24: note: declared here static enum MHD_Result ksr_microhttpd_request(void *cls, ^~~~~~~~~~~~~~~~~~~~~~ microhttpd_mod.c:484:9: warning: 'return' with a value, in function returning void return MHD_YES; ^~~~~~~ microhttpd_mod.c:418:24: note: declared here static enum MHD_Result ksr_microhttpd_request(void *cls, ^~~~~~~~~~~~~~~~~~~~~~ microhttpd_mod.c: In function 'microhttpd_server_run': microhttpd_mod.c:510:17: warning: passing argument 5 of 'MHD_start_daemon' from incompatible pointer type [-Wincompatible-pointer-types] NULL, NULL, &ksr_microhttpd_request, KSR_MICROHTTPD_PAGE, ^~~~~~~~~~~~~~~~~~~~~~~ In file included from microhttpd_mod.c:29: /usr/include/microhttpd.h:2148:1: note: expected 'MHD_AccessHandlerCallback' {aka 'int (*)(void *, struct MHD_Connection *, const char *, const char *, const char *, const char *, long unsigned int *, void **)'} but argument is of type 'void (*)(void *, struct MHD_Connection *, const char *, const char *, const char *, const char *, size_t *, void **)' {aka 'void (*)(void *, struct MHD_Connection *, const char *, const char *, const char *, const char *, long unsigned int *, void **)'} MHD_start_daemon (unsigned int flags, ^~~~~~~~~~~~~~~~ microhttpd_mod.c:515:17: warning: passing argument 5 of 'MHD_start_daemon' from incompatible pointer type [-Wincompatible-pointer-types] NULL, NULL, &ksr_microhttpd_request, KSR_MICROHTTPD_PAGE, ^~~~~~~~~~~~~~~~~~~~~~~ In file included from microhttpd_mod.c:29: /usr/include/microhttpd.h:2148:1: note: expected 'MHD_AccessHandlerCallback' {aka 'int (*)(void *, struct MHD_Connection *, const char *, const char *, const char *, const char *, long unsigned int *, void **)'} but argument is of type 'void (*)(void *, struct MHD_Connection *, const char *, const char *, const char *, const char *, size_t *, void **)' {aka 'void (*)(void *, struct MHD_Connection *, const char *, const char *, const char *, const char *, long unsigned int *, void **)'} MHD_start_daemon (unsigned int flags, ^~~~~~~~~~~~~~~~ make[2]: *** [../../Makefile.rules:100: microhttpd_mod.o] Error 1 make[1]: *** [Makefile:508: modules] Error 1 make[1]: Leaving directory '/root/rpmbuild/BUILD/kamailio-5.8.0-dev1/src' make: *** [Makefile:34: every-module] Error 2 ```