opt: make LWIP_IPV6_SEND_ROUTER_SOLICIT default to LWIP_IPV6

This ensures struct netif doesn't contain 'rs_count' if LWIP_IPV6
is disabled but LWIP_IPV6_SEND_ROUTER_SOLICIT is at its default.

See bug #56509

src/include/lwip/opt.h

@@ -2494,7 +2494,7 @@
  * network startup.
  */
 #if !defined LWIP_IPV6_SEND_ROUTER_SOLICIT || defined __DOXYGEN__
-#define LWIP_IPV6_SEND_ROUTER_SOLICIT   1
+#define LWIP_IPV6_SEND_ROUTER_SOLICIT   LWIP_IPV6
 #endif
 
 /**