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/balloon.c b/drivers/xen/balloon.c
index 5876e1a..1779338 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -39,6 +39,7 @@
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/errno.h>
+#include <linux/module.h>
 #include <linux/mm.h>
 #include <linux/bootmem.h>
 #include <linux/pagemap.h>