unmodified_drivers: enable use of register_oldmem_pfn_is_ram() API

unmodified_drivers: enable use of register_oldmem_pfn_is_ram() API

During the investigation of very slow dump times of guest images in
Amazon EC2 instance, it was discovered that the
register_oldmem_pfn_is_ram() API implemented by the upstream kernel
commit 997c136f518c5debd63847e78e2a8694f56dcf90:

        fs/proc/vmcore.c: add hook to read_from_oldmem() to check
                           for non-ram pages

was not being called.  This was due to the PV driver with the call
to register_oldmem_pfn_is_ram() API was not including the
kernel header file that is used to communicate support of the API in the
kernel.  Fix the issue by including the required header file.

Signed-off-by: Mike Meyer <mike.meyer@teradata.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Olaf Hering <olaf@aepfle.de>