Platform: include windows.h in WindowlessWglApplication.

The flextGL header doesn't include it anymore so I need to include it
explicitly myself.

src/Magnum/Platform/WindowlessWglApplication.h

@@ -29,9 +29,13 @@
  * @brief Class @ref Magnum::Platform::WindowlessWglApplication, macro @ref MAGNUM_WINDOWLESSWGLAPPLICATION_MAIN()
  */
 
-#include "Magnum/OpenGL.h"
+#define WIN32_LEAN_AND_MEAN 1
+#define VC_EXTRALEAN
+#include <windows.h>
 
 #include "Magnum/Magnum.h"
+#include "Magnum/OpenGL.h"
+#include "Magnum/Platform/Context.h"
 
 namespace Magnum { namespace Platform {