MDEV-34567 unit.my_apc always failing on FreeBSD-14

Without the call to my_mutex_init, the mutex attributes
my_fast_mutexattr and my_errorcheck_mutexattr are uninitialized.

Linux tolerates this but FreeBSD doesn't (and segfaults).

We fix for all since the unit text should be testing the
standard mutexes of the system.

unittest/sql/my_apc-t.cc

@@ -190,6 +190,7 @@ int main(int args, char **argv)
   pthread_t request_thr[N_THREADS];
   int i;
 
+  my_mutex_init();
   my_thread_global_init();
 
   mysql_mutex_init(0, &apc_counters_mutex, MY_MUTEX_INIT_FAST);