Fix gstglconfig.h install path
I guess this is a typo: the file was installed as
/usr/local/lib/gstreamer-1.0/include/gst/gl/gstglconfig.h
which is a
weird location for a header file. It probably should better go to
/usr/local/include/gstreamer-1.0/gst/gl/gstglconfig.h
on this system.