Fully qualify header paths when including controllers

so this still works when private headers are installed

examples/dockwidgets/MyTitleBar_CSS.h

@@ -15,7 +15,7 @@
 #pragma once
 
 #include <kddockwidgets/views/TitleBar_qtwidgets.h>
-#include "controllers/TitleBar.h"
+#include "kddockwidgets/controllers/TitleBar.h"
 
 /**
  * @brief Shows how to implement a custom titlebar which uses "Qt StyleSheets".