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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/xen/page.h

···
 #include <linux/pfn.h>
 
 #include <asm/uaccess.h>
+#include <asm/pgtable.h>
 
 #include <xen/features.h>