gst: Add public API for initializing tracer plugin
gst_init() failed to initialize the tracing plugins, because it initializes the tracing system before the tracer plugin is loaded, when statically linked (such as using macro GST_PLUGIN_STATIC_REGISTER.)
https://gitlab.freedesktop.org/gstreamer/gstreamer/blob/master/gst/gst.c#L805-816
If statically linked, GST_PLUGIN_STATIC_REGISTER macro must be called before calling gst_tracing_init() function.
Fixes #329