sysutils/xen-tools: add GNU TLS dependency

The Xen toolstack vncpasswd guest configuration option depends on QEMU
being built with an external crypto library as of QEMU commit:

21407ddf967f crypto: delete built-in DES implementation

Introduce a dependency of the xen-tools package on gnutls, so that the
option continues to work as expected. Otherwise when attempting to use
vncpasswd option guest creation fails due to QEMU not starting.

Reported by: Subbsd <subbsd@gmail.com>
Sponsored by: Cloud Software Group
Approved by: bapt (implicit)