gitignore: add all.config

This is used by kbuild to load preset Kconfig options.  We need to
ignore it, otherwise git clean kills it.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>