gitignore: add /overlay

Similar to /files, /overlay is only useful for modifications not tracked in
the LEDE repository itself.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

.gitignore

@@ -13,6 +13,7 @@
 /feeds
 /feeds.conf
 /files
+/overlay
 /package/feeds
 /package/openwrt-packages
 key-build*