gitignore: ignore patches in OpenWrt root directory

This will have GIT ignore patches in root directory, as created
when using "git format-patch".

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 2c54135598)

.gitignore

@@ -16,6 +16,7 @@
 /overlay
 /package/feeds
 /package/openwrt-packages
+/*.patch
 key-build*
 *.orig
 *.rej