Change GST_GET_TIMESTAMP into gst_util_get_timestamp and replace all uses as we don't have HAVE_POSIX_TIMERS in publi...

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gstclock.h:
* gst/gstdebugutils.c:
* gst/gstinfo.c:
* gst/gstutils.c:
* gst/gstutils.h:
* libs/gst/base/gstbasesink.c:
* tools/gst-launch.c:
Change GST_GET_TIMESTAMP into gst_util_get_timestamp and replace all
uses as we don't have HAVE_POSIX_TIMERS in public headers.
Thanks Tim for spotting.