gst: add missing library suffix in gst-launch helper.
gst: add missing library suffix in gst-launch helper.

AnilRamachandran/aravis,lu-zero/aravis,AnilRamachandran/aravis,AravisProject/aravis,lu-zero/aravis,AravisProject/aravis,lu-zero/aravis,AravisProject/aravis,lu-zero/aravis,AnilRamachandran/aravis,AnilRamachandran/aravis,AravisProject/aravis,AravisProject/aravis,lu-zero/aravis,AnilRamachandran/aravis