gst/: Move declaration of private _gst_foo_initialize() functions into our private header file where they should have...

Original commit message from CVS:
* gst/gst_private.h:
* gst/gstbuffer.h:
* gst/gstevent.h:
* gst/gstformat.h:
* gst/gstmessage.h:
* gst/gstplugin.h:
* gst/gstquery.h:
* gst/gsttaglist.h:
* gst/gstvalue.h:
Move declaration of private _gst_foo_initialize() functions into
our private header file where they should have been all along.