.gitignore: allow debian .patch files

.gitignore: allow debian .patch files

The Ubuntu packaging layout with git-buildpackage assumes a
"debian/patches/" directory with several .patch files in it.

When upstream's .gitignore tells Git to ignore .patch files, we have to
edit that line out downstream. When we forget to do that downstream, it
can lead to missing patches and broken downstream builds.

Allow patches in the /debian/patches directory so it's easier to
maintain an Ubuntu package based on upstream's Git repo.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>