gststructure: Add gst_structure_get_flags method

We don't prevent setting G_TYPE_FLAGS on GstStructure
but no helper method for getting the value.
Add a method similar to gst_structure_get_enum()

Part-of: <gstreamer/gstreamer!2770>
28 jobs for !2770 with struct-flags in 45 minutes and 43 seconds (queued for 2 seconds)
merge request