gst_all_1.gst-plugins-bad: allow building gtksink plugin

Corebird requires gtksink gstreamer plugin to play videos. [1] The plugin,
however, is only built when GTK is available.

This patch adds gtk3 as an optional dependency to gst_all_1.gst-plugins-bad
package, allowing the build of gtksink.

[1]: https://github.com/baedert/corebird/issues/431