Fix "Header" documentation in Core plugin

Change-Id: If3cbe51b47a3acba32429894dfb5154cd573967d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/coreplugin/ioutputpane.cpp

@@ -31,6 +31,7 @@ namespace Core {
 
 /*!
     \class Core::IOutputPane
+    \inheaderfile coreplugin/ioutputpane.h
     \brief The IOutputPane class is an interface for providing \uicontrol Output panes.
 
     \ingroup mainclasses