ptpclock: Use a system clock for the time observations instead of gst_util_get_timest...

ptpclock: Use a system clock for the time observations instead of gst_util_get_timestamp()

They don't necessarily use the same underlying clocks (e.g. on Windows), or
might be configured to a different clock type (monotonic vs. real time clock).

We need the values a clean system clock returns, as those are the values used
by the internal clocks.