[POWERPC] Update lmb.h include protection to ASM_POWERPC

This file was protected by _PPC64_LMB_H, which is confusing, as the
32-bit code also uses the lmb these days.  Changed to
_ASM_POWERPC_LMB_H.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>