Hide Night Color KCM on X

A perceived majority of devs want the KCM to only show up in a Wayland
session, since Night Color functionality is not available under X (here
a user can use Redshift instead).

kcms/nightcolor/kcm_nightcolor.desktop

@@ -13,6 +13,8 @@ X-KDE-ParentApp=kcontrol
 X-KDE-System-Settings-Parent-Category=display
 X-KDE-Weight=120
 
+X-KDE-OnlyShowOnQtPlatforms=wayland
+
 Name=Night Color
 Name[de]=Nachtfarbe
 Comment=Adjust color temperatur at night to reduce eye strain