Enable "combined-angle-lib" for ANGLE builds

As qtlocation's mapboxgl plugin links to QtANGLE if dynamic opengl is
used, we have to keep the combined library being the default until
qtlocation is fixed.
As soon as the qtlocation fix has landed in 5.9.0 this change should be
reverted.

Task-number: QTBUG-60795
Change-Id: I0781695e085a28a8971a6245a352413c0f779025
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/configure.json

@@ -430,6 +430,7 @@
         "combined-angle-lib": {
             "label": "Combined ANGLE Library",
             "autoDetect": false,
+            "enable": "features.angle",
             "condition": "features.angle",
             "output": [ "publicFeature" ]
         },