um: virtio_uml: Disallow modular build

This driver *can* be a module, but then its parameters (socket path)
are untrusted data from inside the VM, and that isn't allowed. Allow
the code to only be built-in to avoid that.

Fixes: 5d38f324 ("um: drivers: Add virtio vhost-user driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>