gstconfig.h: Don't use extern with dllexport
gstconfig.h: Don't use extern with dllexport

GCC emits an error for this with -Werror:

plugin.c:22:1: error: 'gst_plugin_desc' initialized and declared 'extern' [-Werror]

This matches how glib does symbol exporting.

https://bugzilla.gnome.org/show_bug.cgi?id=767463

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer