meson: fix build failure if orc is enabled but none of its users are
meson: fix build failure if orc is enabled but none of its users are

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/issues/778

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/704>

GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good