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>
diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
index cef9b0b..0e867ee 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -46,6 +46,7 @@
 #include <linux/mutex.h>
 #include <linux/io.h>
 #include <linux/slab.h>
+#include <linux/module.h>
 
 #include <asm/page.h>
 #include <asm/pgtable.h>