decproxy: add vp9 mime-type for exposing caps
decproxy: add vp9 mime-type for exposing caps

Change-Id: Id8f35e85be26681be69f2a42aff6a2364cd289a7
Reviewed-on: https://gpro.lgsvl.com/64680
Reviewed-by: Jeongseok Kim <0c03ab7a1b32f29697716389c3f9549845834271@lge.com>
Tested-by: Jeongseok Kim <0c03ab7a1b32f29697716389c3f9549845834271@lge.com>

justinjoy/gst-plugins-cool,justinjoy/gst-plugins-cool