ospf6d: add missing include

Two extern declarations in ospf6_abr.h are based on struct ospf6_route,
which may not be available at the time ospf6_abr.h is included. This may
lead to warnings after including ospf6_abr.h just for the structures
defined in it.
diff --git a/ospf6d/ospf6_abr.h b/ospf6d/ospf6_abr.h
index 5d00c47..86d0028 100644
--- a/ospf6d/ospf6_abr.h
+++ b/ospf6d/ospf6_abr.h
@@ -22,6 +22,9 @@
 #ifndef OSPF6_ABR_H
 #define OSPF6_ABR_H
 
+/* for struct ospf6_route */
+#include "ospf6_route.h"
+
 /* Debug option */
 extern unsigned char conf_debug_ospf6_abr;
 #define OSPF6_DEBUG_ABR_ON() \