Fix a non-deprecated build on MSVC, again.

I attempted in 54394e2c2f and in
54394e2c2f, but still not enough.
Hopefully now?

src/Magnum/Audio/AbstractImporter.cpp

@@ -25,6 +25,7 @@
 
 #include "AbstractImporter.h"
 
+#include <string>
 #include <Corrade/Containers/Array.h>
 #include <Corrade/Containers/EnumSet.hpp>
 #include <Corrade/Containers/Optional.h>

src/Magnum/Audio/AbstractImporter.h

@@ -30,6 +30,7 @@
  */
 
 #include <Corrade/PluginManager/AbstractManagingPlugin.h>
+#include <Corrade/Utility/StlForwardString.h>
 
 #include "Magnum/Magnum.h"
 #include "Magnum/Audio/BufferFormat.h"

src/Magnum/ShaderTools/AbstractConverter.h

@@ -32,6 +32,7 @@
 
 #include <Corrade/Containers/EnumSet.h>
 #include <Corrade/PluginManager/AbstractManagingPlugin.h>
+#include <Corrade/Utility/StlForwardString.h> /** @todo remove once file callbacks are std::string-free */
 
 #include "Magnum/Magnum.h"
 #include "Magnum/ShaderTools/ShaderTools.h"