PE: Fix windows build

Amends c6be4fd2d4.

Change-Id: I423f8ce2f729bd609c250afce1abd02964629e2b
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/taskhub.h

@@ -59,7 +59,7 @@ signals:
     void openTask(const Task &task);
 
 private:
-    friend TaskHub &taskHub();
+    friend PROJECTEXPLORER_EXPORT TaskHub &taskHub();
 
     TaskHub();
     ~TaskHub() final;