xen: Add export.h for THIS_MODULE/EXPORT_SYMBOL to various xen users.

Things like THIS_MODULE and EXPORT_SYMBOL were simply everywhere
because module.h was also everywhere.  But we are fixing the latter.
So we need to call out the real users in advance.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index aceeca7..553a7a2 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -37,6 +37,7 @@
 #include <linux/device.h>
 #include <linux/notifier.h>
 #include <linux/mutex.h>
+#include <linux/export.h>
 #include <linux/completion.h>
 #include <linux/init.h>
 #include <linux/slab.h>