gst/matroska/matroska-mux.c: Also fix up pad templates to indicate that image/jpeg doesn't absolutely require the fra...

Original commit message from CVS:
* gst/matroska/matroska-mux.c: (COMMON_VIDEO_CAPS_NO_FRAMERATE),
(videosink_templ):
Also fix up pad templates to indicate that image/jpeg doesn't
absolutely require the framerate property to be set (#504081).