gst-plugins-ugly: fix dependencies for mad MP3 decoder plug-in

The mad plug-in depends on libid3tag. This dependency used to be
implicit by selecting the id3tag plug-in. Now that this plug-in
is not any longer part of gst-plugins-ugly we need an explicit
dependency on libid3tag.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>