package/libid3tag: fix id3tag.pc

Add -lz to id3tag.pc, this fix is needed to be able to use pkg-config in
madplay to find id3tag dependencies

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>