Fix warning in libc_replacements.c

Author: igrr

Diff for: cores/esp8266/libc_replacements.c

@@ -26,6 +26,7 @@
 #include <stddef.h>
 #include <stdarg.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <math.h>
 #include <limits.h>
 #include <errno.h>