package/spice: Only builds on x86

The spice configure script explicitly checks for x86 or x86-64, so
disable it on other architectures

Fixes
http://autobuild.buildroot.org/results/7f7e2d46542a685b93211f76ad063f8b2ba7b5e5/build-end.log



Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>