DiffEditor: Make DiffEditorGuiController internal

Change-Id: I657085535b7f141c97bc3d054e9edfd236209981
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/diffeditor/sidebysidediffeditorwidget.cpp

@@ -72,6 +72,8 @@ using namespace Utils;
 
 namespace DiffEditor {
 
+using namespace Internal;
+
 ////////////////////////
 /*
 class MultiHighlighter : public SyntaxHighlighter