wasm: export qstdweb::readDataTransfer()

Fix shared libraries build.

Change-Id: I7bf1c7d333b7d039905ee77cf954ba46549993d3
Reviewed-by: Mikołaj Boc <Mikolaj.Boc@qt.io>

src/corelib/platform/wasm/qstdweb_p.h

@@ -207,7 +207,7 @@ namespace qstdweb {
     {
     };
 
-    std::shared_ptr<CancellationFlag>
+    Q_CORE_EXPORT std::shared_ptr<CancellationFlag>
     readDataTransfer(emscripten::val webObject, std::function<QVariant(QByteArray)> imageReader,
                      std::function<void(std::unique_ptr<QMimeData>)> onDone);
 }