xen, x86/headers: Add <linux/device.h> dependency to <asm/xen/page.h>

The following patch (not upstream yet):

  "x86/boot/e820: Remove spurious asm/e820/api.h inclusions"

Removed the (spurious) <asm/e820.h> include line from <asm/pgtable.h> to
reduce header file dependencies - but a Xen header has (unintentionally)
learned to rely on the indirect inclusion of <linux/device.h>.

This resulted in the following (harmless) build warning:

   arch/x86/include/asm/xen/page.h:302:7: warning: 'struct device' declared inside parameter list

Include <linux/device.h> explicitly.

No change in functionality.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Juergen Gross <jgross@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: <stefano.stabellini@eu.citrix.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/xen/page.h

@@ -6,6 +6,7 @@
 #include <linux/spinlock.h>
 #include <linux/pfn.h>
 #include <linux/mm.h>
+#include <linux/device.h>
 
 #include <linux/uaccess.h>
 #include <asm/page.h>