configure.ac: Depend on libldns via ldns.pc instead of libldns.pc libldns.pc was apparently the filename used by older Debian ldns packages. Debian's ldns 1.8.3-1 package switched to solely installing the upstream provided ldns.pc file and stopped installing libldns.pc.
Without this change, dnstap-ldns will FTBFS since it won't be able to find libldns.pc.
When submitting this patch upstream, the configure script should probably support both libldns.pc as well as ldns.pc, which is probably something like:
