x86: add stub init_gbpages() for 32-bit init_memory_mapping()

Impact: cleanup

This patch adds an empty static inline init_gbpages() for the 32-bit
version of init_memory_mapping() making both versions identical.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1236257708-27269-13-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>