[PATCH] Add offset.h to dontdiff

include/asm/offset.h is a generated file on x86_64 and mips.  Let's add it
to Documentation/dontdiff.

Signed-off-by: Michal Schmidt <xschmi00@stud.feec.vutbr.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>