fix: add missing stdlib.h include in heartbeat.c

The code calls free() without including <stdlib.h>, which triggers
implicit declaration warnings during the build.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

main/heartbeat/heartbeat.c

@@ -3,6 +3,7 @@
 #include "bus/message_bus.h"
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <stdbool.h>
 #include <ctype.h>