Fix: Multiple implicitly declared functions

https://github.com/knopwob/dunst/issues/267

dunst.h

@@ -3,6 +3,7 @@
 #pragma once
 
 #include <glib.h>
+#include <stdio.h>
 #include <stdbool.h>
 
 #include "x.h"

settings.c

@@ -1,6 +1,8 @@
 /* copyright 2013 Sascha Kruse and contributors (see LICENSE for licensing information) */
 
 #include <glib.h>
+#include <stdlib.h>
+#include <string.h>
 
 #ifndef STATIC_CONFIG
 #include <basedir.h>