gnome-control-center.desktop: Ensure we're in the menu

If we're not categorized, the shell won't pick us up, and that's
a problem for application tracking.  Per IRC discussion, go
into System Settings for now.

shell/gnome-control-center.desktop.in.in

@@ -5,7 +5,7 @@ Exec=gnome-control-center --overview
 Terminal=false
 Type=Application
 StartupNotify=true
-Categories=GNOME;GTK;Settings;
+Categories=GNOME;GTK;System;
 OnlyShowIn=GNOME;
 X-GNOME-Bugzilla-Bugzilla=GNOME
 X-GNOME-Bugzilla-Product=gnome-control-center