gst1-plugins-good: add the y4m plugin package

gst1-plugins-good: add the y4m plugin package

In 1.28 upstream merged the y4mdec element from gst-plugins-bad into the
gst-plugins-good y4m plugin, which now registers both y4mdec and y4menc
from a single libgsty4m. The Makefile has passed -Dy4m= since 1.16.2,
but the package line stayed commented out, so the plugin was never
built.

Enable it, so the y4mdec element dropped from gst-plugins-bad in this
series stays available. Anyone who selected gst1-mod-y4mdec needs to
select gst1-mod-y4m instead.

Signed-off-by: Alexandru Ardelean <redacted>