pfring: workaround potential librt deps

It seems some version of pfring needs to be link with librt.

Ticket: https://redmine.openinfosecfoundation.org/issues/1026

PR builds:
- PR build: https://buildbot.suricata-ids.org/builders/regit/builds/74
- PR pcaps: https://buildbot.suricata-ids.org/builders/regit-pcap/builds/12