Test suite: Explicitly link gen_ocsp_index against libgnutls
Test suite: Explicitly link gen_ocsp_index against libgnutls

This is necessary to successfully link gen_ocsp_index on Fedora 23,
shouldn't hurt on other systems.

airtower-luna/mod_gnutls,airtower-luna/mod_gnutls,airtower-luna/mod_gnutls