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>

drivers/xen/platform-pci.c

@@ -45,7 +45,7 @@ static unsigned long platform_mmio_alloc;
 static unsigned long platform_mmiolen;
 static uint64_t callback_via;
 
-unsigned long alloc_xen_mmio(unsigned long len)
+static unsigned long alloc_xen_mmio(unsigned long len)
 {
 	unsigned long addr;