gstreamer: Fix build on 32bit arches with 64bit time_t

Fixes

| ../gstreamer-1.18.1/gst/gstsystemclock.c:330:1: error: static_assert failed due to requirement 'size
of(struct _GstClockEntryPThread) <= sizeof(struct _GstClockEntryImpl)' "Expression evaluates to false"
| G_STATIC_ASSERT (sizeof (GstClockEntryImpl) <=                                                      | ^                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>