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/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index 32417b5..c3c7cd1 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -42,6 +42,7 @@
 #include <linux/fcntl.h>
 #include <linux/mm.h>
 #include <linux/notifier.h>
+#include <linux/export.h>
 
 #include <asm/page.h>
 #include <asm/pgtable.h>