build: Enable Gstreamer plugin by default

This is coherent with Autoconf.
We want to have this plugin enabled by default.
Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Kevin Pouget <kpouget@redhat.com>

meson_options.txt

 option('gst-plugin',
        type : 'feature',
+       value : 'enabled',
        description : 'Enable GStreamer based plugin')
 
 option('unittests',