Staging: hv: force hyper-v drivers to be built as a module

Right now they can not be built into the kernel due to global
symbol name conflicts that this code is causing.

Thanks to Nicolas Palix <npalix@diku.dk> for pointing this out.

Reported-by: Nicolas Palix <npalix@diku.dk>
Cc: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>