Add missing includes for implicitly declared functions
https://bugs.gentoo.org/875479
--- a/ping-apollo.c	2025-02-14 00:18:28.905559670 +0400
+++ b/ping-apollo.c	2025-02-14 00:19:59.617012294 +0400
@@ -9,6 +9,8 @@
 #include <sys/time.h>
 #include <unistd.h>
 #include <linux/serial.h>
+#include <stdlib.h>
+#include <string.h>
 
 #define DEFAULT_DEVICE "/dev/ttyS0"
 #define MAXCHARS 128