[Gui] ViewProviderLink etc.: remove unused includes

- also put headers to PreCompiled.h

src/Gui/ViewProviderLinkPyImp.cpp

@@ -26,13 +26,13 @@
 # include <sstream>
 #endif
 
-#include "Gui/ViewProviderLink.h"
-#include <App/DocumentObject.h>
 #include <Base/PlacementPy.h>
+
 // inclusion of the generated files (generated out of ViewProviderLinkPy.xml)
 #include "ViewProviderLinkPy.h"
 #include "ViewProviderLinkPy.cpp"
 
+
 using namespace Gui;
 
 // returns a string which represents the object e.g. when printed in python