Disable the effect video button if gstreamer is missing.

--- a/kcmkwin/kwineffects/package/contents/ui/Effect.qml	2019-03-22 23:06:45.172699544 +0100
+++ b/kcmkwin/kwineffects/package/contents/ui/Effect.qml	2019-03-22 23:07:30.435043637 +0100
@@ -111,7 +111,7 @@
 
         QtControls.Button {
             icon.name: "video"
-            visible: model.VideoRole.toString() !== ""
+            visible: false
 
             onClicked: videoItem.showHide()
         }