drivers: xen: Mark function as static in platform-pci.c

Mark function as static in xen/platform-pci.c because it is not used
outside this file.

This eliminates the following warning in xen/platform-pci.c:
drivers/xen/platform-pci.c:48:15: warning: no previous prototype for ‘alloc_xen_mmio’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>