Fix static warning message.

[jpereira]

eg:

pam_radius_auth.c:1980:16: warning: no previous prototype for function 'pam_sm_acct_mgmt' [-Wmissing-prototypes]
 1980 | PAM_EXTERN int pam_sm_acct_mgmt(UNUSED pam_handle_t *pamh, UNUSED int flags, UNUSED int argc, UNUSED CONST char **argv)
  1 Fix warning assignment discards qualifiers
      |                ^
pam_radius_auth.c:1980:12: note: declare 'static' if the function is not intended to be used outside of this translation unit
 1980 | PAM_EXTERN int pam_sm_acct_mgmt(UNUSED pam_handle_t *pamh, UNUSED int flags, UNUSED int argc, UNUSED CONST char **argv)
      |            ^
      |            static