gstreamer-uninstalled: add path for OpenCV haar cascade files
gstreamer-uninstalled: add path for OpenCV haar cascade files

Some OpenCV plugins use haar cascade files that are included in the
GStreamer sources. To be able to use these from uninstalled, they need
to be found through an environment variable.

Adding this environment variable pointing to haar cascade files to
gst-uninstalled.

jpxiong/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpxiong/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer