x86: asm-x86/pgtable.h: fix compiler warning

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: xen-devel <xen-devel@lists.xensource.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/pgtable.h

@@ -425,6 +425,8 @@ static inline void native_set_pte_at(struct mm_struct *mm, unsigned long addr,
  * race with other CPU's that might be updating the dirty
  * bit at the same time.
  */
+struct vm_area_struct;
+
 #define  __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
 extern int ptep_set_access_flags(struct vm_area_struct *vma,
 				 unsigned long address, pte_t *ptep,