linux gntdev: Mark x86-specific code as such with ifdef.

Signed-off-by: Keir Fraser <keir@xensource.com>

linux-2.6-xen-sparse/drivers/xen/gntdev/gntdev.c

@@ -518,12 +518,14 @@ static int gntdev_mmap (struct file *flip, struct vm_area_struct *vma)
 	 */
 	vma->vm_flags |= VM_DONTCOPY;
 
+#ifdef CONFIG_X86
 	/* This flag ensures that the page tables are not unpinned before the
 	 * VM area is unmapped. Therefore Xen still recognises the PTE as
 	 * belonging to an L1 pagetable, and the grant unmap operation will
 	 * succeed, even if the process does not exit cleanly.
 	 */
 	vma->vm_mm->context.has_foreign_mappings = 1;
+#endif
 
 	for (i = 0; i < size; ++i) {