gst/gstutils.h: Add prototype for _get_type() function to GST_BOILERPLATE_FULL macro, so that gcc doesn't complain if...

Original commit message from CVS:
Reviewed by: Tim-Philipp Müller  <tim at centricular dot net>
* gst/gstutils.h:
Add prototype for _get_type() function to GST_BOILERPLATE_FULL
macro, so that gcc doesn't complain if the -Wmissing-prototypes
compiler switch is being used (#325429).