Explicitly link against gnutls
booth 1.2 supports gnutls and prefers it over gcrypt when searching for a encryption library to link against. Since pacemaker-dev pulls libgnutls-dev as a dependency, gnutls is available during build time and that is what we are linking against now. This seems to have happened by chance, since d/control still includes a B-D entry for gcrypt.
The proposed changes replace the B-D on gcrypt for gnutls and explicitly links against gnutls to avoid changing the library this package uses for encryption in case the upstream project changes the autodetection order to prefer something else other than gnutls.