gst: Add public API for initializing tracer plugin

Open Yeongjin Jeong requested to merge Yeongjin-Jeong/gstreamer:public-tracer-init into master

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.)

If statically linked, GST_PLUGIN_STATIC_REGISTER macro must be called before calling gst_tracing_init() function.

Fixes #329

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