xen: xen/page.h compile fix

Fix:
linux/include/xen/page.h: In function mfn_pte:
linux/include/xen/page.h:149: error: __supported_pte_mask undeclared (first use in this function) linux/include/xen/page.h:149: error: (Each undeclared identifier is reported only once linux/include/xen/page.h:149: error: for each function it appears in.)

---
 include/xen/page.h |    1 +
 1 file changed, 1 insertion(+)

===================================================================

@@ -4,6 +4,7 @@
 #include <linux/pfn.h>
 
 #include <asm/uaccess.h>
+#include <asm/pgtable.h>
 
 #include <xen/features.h>