Core: Fix missing include

Change-Id: I08266f0aaa1d7b33f8273ed585b8c97df2e85038
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/editormanager/ieditorfactory.h

@@ -33,6 +33,8 @@
 #include <QObject>
 #include <QStringList>
 
+#include <functional>
+
 namespace Core {
 
 class IEditor;