enable imx6 specific QSGVideoNode based on a config test

This way it also works custom mkspecs

Change-Id: I7414aac775b797da3487e9625c7212486118b532
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>

src/plugins/videonode/videonode.pro

@@ -1,5 +1,5 @@
 TEMPLATE = subdirs
 
-*imx6* {
+config_gpu_vivante {
     SUBDIRS += imx6
 }