Ok, so an update, in order for auth_radius to work with freeradius-client `VENDOR()` must be defined in the headers.
Currently `VENDOR()` is being defined in the radcli library as `#define VENDOR(x) (((x) >> 16) & 0xffff)`(https://github.com/radcli/radcli/blob/f0d1a1704595d62182d6a03c980ed07e906374...) but is missing from freeradius-client.
By adding the line in `misc_radius/radius.h` there are no more implicit declarations and the module is compiled properly by using `FREERADIUS=1 make`, and properly works (headers are being added with the right attribute codes).
I think that the proper place to add the VENDOR definition is here:
https://github.com/kamailio/kamailio/blob/e8ce5cff74c3d53df361e42746bb4a7c1f...