MDEV-24577: Fix warnings generated during compilation of plugin/auth_pam/testing/pam_mariadb_mtr.c on FreeBSD

Compiler warnings generated on building MariaDB server for BSD has the same
reason as in case building is performed on MacOS. Both platforms do use
clang as a C/C++ compiler. So, fix the compiler warnings in case the compiler
is clang doesn't matter what kind of building platform do we use for building.

This is a follow-up patch for the following bug reports:
  MDEV-23564: CMAKE failing due to deprecated Apple GSS method
  MDEV-23935: Fix warnings generated during compilation of
              plugin/auth_pam/testing/pam_mariadb_mtr.c on MacOS