x86: remove offsets.h from .gitignore and dontdiff

Commit 77d1a499 ("x86, boot: make
symbols from the main vmlinux available") removed all traces of
offsets.h from the tree. Remove its entries in dontdiff and x86/boot's
.gitignore file too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>