audio/gstreamer-plugins-flac: rebuild fails chasing audio/flac due to shlib in LIB_DEPENDS

[Barbara Guida]

After upgrading audio/flac, rebuilding audio/gstreamer-plugins-flac will
fail trying to reinstall flac. That's because in
multimedia/gstreamer-plugins/Makefile.common there are shlib version
still hardcoded on several LIB_DEPENDS lines.
Maybe not all of them could be simply removed (e.g. xml2.5 for
textproc/libxml2), but I guess that it could be possible for most of them.

Cheers
Barbara