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 <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Reviewed-by: David Vrabel <[email protected]>

Author: rashika

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;