.gitignore: ignore more patch related files

Patches shouldn't be committed to the top level, and that is where
`git format-patch` typically operates, so ignore that stuff.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>