Backport of "gstelements_private: sync gst_buffer_get_flags_string() with new flags" into 1.16
Backport of !183 (merged) "gstelements_private: sync gst_buffer_get_flags_string() with new flags".
Commits:
- f2d37d51 gstelements_private: sync gst_buffer_get_flags_string() with new flags