Link against libz if compiling with libid3tag support.

libid3tag depends on libz. On Linux, libz is automatically linked to
para_server and para_afh but at least on FreeBSD compilation fails
due to undefined references to "uncompress" and "compress2".

This patch explicitly adds -lz to $server_ld_flags and $afh_ldflags
if libid3tag was detected.