Move including of gnutls/gnutls.h in qemu/qemu_domain.c under the
"ifdef WITH_GNUTLS" check because otherwise it fails like this:

  CC       qemu/libvirt_driver_qemu_impl_la-qemu_domain.lo
qemu/qemu_domain.c:50:10: fatal error: 'gnutls/gnutls.h' file not found

in case if gnutls is not installed on the system.