Fix configuring Prelude with -Werror

Running with:

CFLAGS="-Werror" ./configure

would fail when configuring libprelude because of an unused-result
warning. Ignore that one warning.

Passes both new regression tests:
https://buildbot.suricata-ids.org/builders/ken-tilera-pcap/builds/3
https://buildbot.suricata-ids.org/builders/ken-tilera/builds/66