porting: Add missing include in linux example

main.c: In function ‘main’:
main.c:65:33: warning: implicit declaration of function ‘atoi’ [-Wimplicit-function-declaration]
   65 |         ble_hci_sock_set_device(atoi(argv[1]));
      |                                 ^~~~

porting/examples/linux/main.c

@@ -19,6 +19,7 @@
 
 #include <stdbool.h>
 #include <stdint.h>
+#include <stdlib.h>
 
 #include <pthread.h>
 #include "nimble/nimble_npl.h"