Bug#788942: Dependency on non-multiarch libepoxy0 breaks multiarch

[sacrificial-spam-address at horizon.com]

Package: libgtk-3-0
Version: 3.16.4-2

I can't upgrade libgtk-3-0:i386 and libgtk-3-0:amd64, because both want
the corresponding libepoxy0, and it's not multiarch, so that's not possible.

The fix might be to make libepoxy multiarch (it already puts its
library in the right directory), but adding that dependency before
fixing libepoxy is a bug in gtk+3.0.