certs: add 'x509_revocation_list' to gitignore

Commit d1f04410 ("certs: Add ability to preload revocation certs")
created a new generated file for revocation certs, but didn't tell git
to ignore it.  Thus causing unnecessary "git status" noise after a
kernel build with CONFIG_SYSTEM_REVOCATION_LIST enabled.

Add the proper gitignore magic.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>