explicitly include <stdlib.h> in case it isn't pulled in by lwIP

setenv() requires <stdlib.h> which is included via lwIP. Make it explicit in case someone uses the timezone code in a different context.

Author: Martin Crossley

pico_w/wifi/ntp_system_time/ntp_system_time.c

@@ -5,6 +5,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>     // needed for setenv(), although also included by lwIP
 #include "pico/stdlib.h"
 #include "pico/cyw43_arch.h"
 #include "lwip/apps/sntp.h"