Forward-declare std::string.

Because we can.

src/Magnum/Mesh.cpp

@@ -25,6 +25,7 @@
 
 #include "Mesh.h"
 
+#include <string>
 #include <Corrade/Containers/ArrayView.h>
 #include <Corrade/Utility/Assert.h>
 #include <Corrade/Utility/Debug.h>

src/Magnum/Mesh.h

@@ -29,7 +29,7 @@
  * @brief Enum @ref Magnum::MeshPrimitive, @ref Magnum::MeshIndexType, function @ref Magnum::meshIndexTypeSize()
  */
 
-#include <string>
+#include <Corrade/Utility/StlForwardString.h>
 
 #include "Magnum/Magnum.h"
 #include "Magnum/visibility.h"