misc: Add missing include for atoi

pcu_vty.c:128:21: warning: implicit declaration of function 'atoi' is invalid in C99 [-Wimplicit-function-declaration]
        bts->fc_interval = atoi(argv[0]);
                           ^

src/pcu_vty.c

@@ -2,6 +2,7 @@
 
 
 #include <stdint.h>
+#include <stdlib.h>
 #include <osmocom/vty/logging.h>
 #include <osmocom/core/linuxlist.h>
 #include "pcu_vty.h"