Fix validation

--- a/gksu-properties/gksu-properties.desktop.in	2017-09-03 16:41:57.872594752 +0200
+++ b/gksu-properties/gksu-properties.desktop.in	2017-09-03 16:44:40.835919489 +0200
@@ -3,9 +3,9 @@
 _Name=Privilege granting
 _Comment=Configure behavior of the privilege-granting tool
 Exec=gksu-properties
-Icon=gksu.png
+Icon=gksu
 StartupNotify=true
 Terminal=false
 Type=Application
-Categories=GNOME;GTK;Application;Settings;AdvancedSettings;
+Categories=GNOME;GTK;Settings;
 OnlyShowIn=GNOME;XFCE;