Add missing header includer for C libraries.

src/Graphics.h

@@ -2,6 +2,8 @@
 #define GRAPHICS_H
 
 #include <string>
+#include <cstdlib>
+#include <cstring>
 #if defined(OGLR)
 #include "OpenGLHeaders.h"
 #endif

src/interface/Engine.cpp

@@ -1,8 +1,8 @@
 #include <iostream>
+#include <stack>
+#include <cstdio>
 
 #include "Config.h"
-#include <stack>
-
 #include "interface/Window.h"
 #include "interface/Platform.h"
 #include "interface/Engine.h"

src/simulation/Simulation.h

@@ -8,6 +8,7 @@
 #ifndef SIMULATION_H_
 #define SIMULATION_H_
 #include <cstring>
+#include <cstddef>
 #include "Config.h"
 #include "Renderer.h"
 #include "Graphics.h"