x86, hypervisor: add missing <linux/module.h>

EXPORT_SYMBOL() needs <linux/module.h> to be included; fixes modular
builds of the VMware balloon driver, and any future modular drivers
which depends on the hypervisor.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Greg KH <greg@kroah.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: Alok Kataria <akataria@vmware.com>
Cc: Ky Srinivasan <ksrinivasan@novell.com>
Cc: Dmitry Torokhov <dtor@vmware.com>
LKML-Reference: <4BE49778.6060800@zytor.com>