xenbus: fix compile failure on ARM with Xen enabled

commit 45e27161 upstream.

Adding an include of linux/mm.h resolves this:
	drivers/xen/xenbus/xenbus_client.c: In function ‘xenbus_map_ring_valloc_hvm’:
	drivers/xen/xenbus/xenbus_client.c:532:66: error: implicit declaration of function ‘page_to_section’ [-Werror=implicit-function-declaration]

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>