FD_ZERO: Fix a compilation warning

Definition of memset is not found if string.h is not included anywhere

Signed-off-by: Piyush Shah <piyush@espressif.com>

components/lwip/include/lwip/lwip/sockets.h

@@ -39,6 +39,7 @@
 #if LWIP_SOCKET /* don't build if not configured for use in lwipopts.h */
 
 #include <stddef.h> /* for size_t */
+#include <string.h> /* for FD_ZERO */
 
 #include "lwip/ip_addr.h"
 #include "lwip/err.h"