Fix some warnings discovered by compiling the unixsim under cygwin (gcc pedantic)

src/include/lwip/api_msg.h

@@ -39,13 +39,12 @@
 #include "lwip/ip_addr.h"
 #include "lwip/err.h"
 #include "lwip/sys.h"
+#include "lwip/igmp.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-enum netconn_igmp;
-
 /* IP addresses and port numbers are expected to be in
  * the same byte order as in the corresponding pcb.
  */