Clean exported headers of the Core plugin.

Change-Id: I26472d568844d5fee62323e01f5c5c12082d5450
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/coreplugin/coreplugin.cpp

@@ -31,6 +31,7 @@
 #include "coreplugin.h"
 #include "designmode.h"
 #include "editmode.h"
+#include "idocument.h"
 #include "helpmanager.h"
 #include "mainwindow.h"
 #include "modemanager.h"