ofp-ct: Add missing struct declaration.

The compilation would fail if ofp-ct.h was included in module that
didn't include dynamic-string.h before the ofp-ct.h include.

Signed-off-by: Ales Musil <amusil@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

Author: almusil

include/openvswitch/ofp-ct.h

@@ -24,6 +24,8 @@
 
 #include "openflow/nicira-ext.h"
 
+struct ds;
+
 #ifdef __cplusplus
 extern "C" {
 #endif