x86, alternative: Add header guards to <asm/alternative-asm.h>

Add header guards to protect <asm/alternative-asm.h> against multiple
inclusion.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348256595-29119-6-git-send-email-hpa@linux.intel.com