pthread: Fix pthread_rwlock_init return error

The return should be positive like in other parts of pthread code

libs/libc/pthread/pthread_rwlock.c

@@ -55,7 +55,7 @@ int pthread_rwlock_init(FAR pthread_rwlock_t *lock,
 
   if (attr != NULL)
     {
-      return -ENOSYS;
+      return ENOSYS;
     }
 
   lock->num_readers       = 0;