posix: re-introduce PTHREAD_RWLOCK_INITIALIZER

Rename the POSIX_RWLOCK_INITIALIZER back to PTHREAD_RWLOCK_INITIALIZER.
This was changed in 70e2b02c, but PTHREAD_RWLOCK_INITIALIZER is the
standard name used by external libraries.

Change it back to restore compatibility.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>