deskutils/spice-gtk is brought in to gnome3 via net/vinagre. However,
the spice-gtk website states that only little-endian is supported.

Mark spice-gtk as such, and change vinagre to only have the dependency
for little-endian. This will help fix the gnome3 build on powerpc64.

PR: 233464
Reviewed by: xxjack12xx at gmail dot com
Approved by: portmgr (tier-2 blanket)