deskutils/spice-gtk: add missing LIB_DEPENDS, USES+=gnome

spice-gtk picks up libva when present with no option to disable it easily,
so make it a requisite for a consistent dependency situation in bare-metal
and jail builds.

The FreeBSD ports rig complains that USE_GNOME requires USES=gnome,
so add that.

PR: 237264
Approved by: xxjack12xx@gmail.com (maintainer)