Fixed a minor warning and a compile error

src/SFML/Window/GlResource.cpp

@@ -38,7 +38,6 @@ namespace
 {
     // OpenGL resources counter and its mutex
     unsigned long count = 0;
-    bool initialized = false;
     sf::Mutex mutex;
 }
 

src/SFML/Window/WindowImpl.cpp

@@ -28,6 +28,7 @@
 #include <SFML/Window/WindowImpl.hpp>
 #include <SFML/Window/Event.hpp>
 #include <SFML/Window/JoystickManager.hpp>
+#include <SFML/System/Sleep.hpp>
 #include <algorithm>
 #include <cmath>