highgui: fix segfault on CvCapture_GStreamer::retrieveFrame

CvCapture_GStreamer::retrieveFrame assumes that RGB videos are 24BPP.
This is not necesarily the case, unless we explicitly tell GStreamer
that we want 24BPP RGB streams.

Adding bpp=(int)24 to the appsink caps.
Merge pull request #2721 from zarrabeitia:2.4