matroskamux: make GstMatroskamuxPad get_type() function thread-safe
matroskamux: make GstMatroskamuxPad get_type() function thread-safe

https://bugzilla.gnome.org/show_bug.cgi?id=739722

jpakkane/gstreamer-plugins-good,jpakkane/gstreamer-plugins-good,ndufresne/gst-plugins-good,wkatsak/gst-plugins-good,wkatsak/gst-plugins-good,vatavuserban/gst-plugins-good,ndufresne/gst-plugins-good,jpakkane/gstreamer-plugins-good,jpakkane/gstreamer-plugins-good,vatavuserban/gst-plugins-good,ndufresne/gst-plugins-good,vatavuserban/gst-plugins-good,vatavuserban/gst-plugins-good,wkatsak/gst-plugins-good,wkatsak/gst-plugins-good,ndufresne/gst-plugins-good