xen/p2m: Fix module linking error.

Fixes:
ERROR: "m2p_find_override_pfn" [drivers/block/xen-blkfront.ko] undefined!
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/p2m.c

@@ -507,3 +507,4 @@ unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn)
 
 	return ret;
 }
+EXPORT_SYMBOL_GPL(m2p_find_override_pfn);