Trade: fix deprecated build on MSVC.

As the CI only uses Linux to test non-deprecated builds, this was
unfortunately not caught -- there std::string is forward-declared in
<iosfwd>, which is included by Debug.h, which is transitively included
by Pointer. On MSVC however a full <string> has to be included always.

Co-authored-by: EhWhoAmI <zyunlam@gmail.com>

src/Magnum/Trade/AbstractImageConverter.h

@@ -35,6 +35,12 @@
 #include "Magnum/Trade/Trade.h"
 #include "Magnum/Trade/visibility.h"
 
+#ifdef MAGNUM_BUILD_DEPRECATED
+/* So deprecated APIs taking a std::string don't fail to compile */
+/** @todo remove once they are gone */
+#include <Corrade/Utility/StlForwardString.h>
+#endif
+
 namespace Magnum { namespace Trade {
 
 /**

src/Magnum/Trade/AbstractSceneConverter.h

@@ -36,6 +36,12 @@
 #include "Magnum/Trade/Trade.h"
 #include "Magnum/Trade/visibility.h"
 
+#ifdef MAGNUM_BUILD_DEPRECATED
+/* So deprecated APIs taking a std::string don't fail to compile */
+/** @todo remove once they are gone */
+#include <Corrade/Utility/StlForwardString.h>
+#endif
+
 namespace Magnum { namespace Trade {
 
 /**