debian: change Priority to extra (makes more sense)

debian/control

@@ -1,6 +1,6 @@
 Source: i3lock
 Section: utils
-Priority: optional
+Priority: extra
 Maintainer: Michael Stapelberg <michael@stapelberg.de>
 DM-Upload-Allowed: yes
 Build-Depends: debhelper (>= 5), libx11-dev, libpam0g-dev, libxext-dev
@@ -9,7 +9,7 @@ Homepage: http://i3.zekjur.net/i3lock/
 
 Package: i3lock
 Architecture: any
-Priority: optional
+Priority: extra
 Section: x11
 Depends: ${shlibs:Depends}, ${misc:Depends}
 Description: a slightly improved version of slock