Fix bug #49125: Need a way to iterate multicast groups for MAC filtering

-> let list heads be a public symbol

src/include/lwip/igmp.h

@@ -88,6 +88,9 @@ struct igmp_group {
   u8_t               use;
 };
 
+/* Head of IGMP group list */
+extern struct igmp_group* igmp_group_list;
+
 /*  Prototypes */
 void   igmp_init(void);
 err_t  igmp_start(struct netif *netif);

src/include/lwip/mld6.h

@@ -72,6 +72,9 @@ struct mld_group {
   u8_t               use;
 };
 
+/* Head of MLD group list */
+extern struct mld_group* mld_group_list;
+
 #define MLD6_TMR_INTERVAL              100 /* Milliseconds */
 
 err_t  mld6_stop(struct netif *netif);