Use forward declaration header instead.

src/Magnum.h

@@ -28,7 +28,7 @@
  * @brief Forward declarations for Magnum namespace
  */
 
-#include <corradeConfigure.h>
+#include <Utility/Utility.h>
 
 #include "Math/Math.h"
 #include "Types.h"
@@ -38,14 +38,6 @@
 typedef unsigned int GLenum; /* Needed for *Format and *Type enums */
 #endif
 
-namespace Corrade {
-    namespace Utility {
-        class Debug;
-        class Warning;
-        class Error;
-    }
-}
-
 namespace Magnum {
 
 /** @todoc Remove `ifndef` when Doxygen is sane again */