vfio-platform: Add COMPILE_TEST to VFIO_PLATFORM

x86 can build platform bus code too, so vfio-platform and all the platform
reset implementations compile successfully on x86.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Message-Id: <2-v1-df057e0f92c3+91-vfio_arm_compile_test_jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>