gtk: examples: #define GST_USE_UNSTABLE_API and link with X11_LIBS
gtk: examples: #define GST_USE_UNSTABLE_API and link with X11_LIBS

X11_LIBS is needed for XInitThreads() and without the #define we get
warnings about the GL API being still unstable.

GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good