niimaqsrc: add UNIX epoch reference timestamp for absolute timestamping

Requires GStreamer 1.14 and a downstream element that can use it.

sys/niimaq/gstniimaq.h

@@ -73,6 +73,10 @@ struct _GstNiImaqSrc {
   gboolean session_started;
 
   GAsyncQueue *time_queue;
+
+  GstClock *clock;
+  GstClockTime stream_base;
+  GstClockTime unix_base;
 };
 
 struct _GstNiImaqSrcClass {