protobuf: host-protobuf can only be built on certain architectures

The host-protobuf package can only build on certain architectures. For
example, host-protobuf does not build on PowerPC platforms, causing
build failures on the autobuilders. Since we don't really care about
build platforms other than x86 and x86-64, this commit restricts the
build of host-protobuf to these architectures only, even though if
technically it could build on ARM and MIPS.

Fixes:

 http://autobuild.buildroot.org/results/9b9/9b9b20b0e4694b11425cdc2382650cdc2774e400/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>