Link libvirt_util.la with gnutls

Link libvirt_util.la with gnutls

The util/vircrypto.c file uses gnutls, so we must directly link
libvirt_util.la with gnutls to avoid errors on OS which do not
resolve symbols against indirectly linked libraries.

This fixes a build failure on Ubuntu Trusty

  CCLD     storagevolxml2argvtest
/usr/bin/ld: ../src/.libs/libvirt_util.a(libvirt_util_la-vircrypto.o): undefined reference to symbol 'gnutls_strerror@@GNUTLS_1_4'

//usr/lib/x86_64-linux-gnu/libgnutls.so.26: error adding symbols: DSO missing from command line

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>