xen: Add module.h to modular drivers/xen users.

Previously these drivers just got module.h implicitly, but we
are cleaning that up and it will be no longer.  Call out the
real users of it.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

drivers/xen/xen-pciback/conf_space.c

@@ -10,6 +10,7 @@
  */
 
 #include <linux/kernel.h>
+#include <linux/module.h>
 #include <linux/pci.h>
 #include "pciback.h"
 #include "conf_space.h"