pkgconfig: fix missing gst-tag Requires

pkgconfig: fix missing gst-tag Requires

Promote deps gstreamer-tag to Requires for pbutils and add missing
gstreamer-tag Requires for gst-audio. Uninstalled builds would fail
with:
stage/usr/local/lib/x86_64-linux-gnu/libgstpbutils-1.0.so: undefined reference to `gst_tag_list_to_vorbiscomment_buffer'
[...]
stage/usr/local/lib/x86_64-linux-gnu/libgstaudio-1.0.so: undefined reference to `gst_tag_register_musicbrainz_tags'