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.)
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/xen/page.h

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