Delete GUI.h umbrella include following 8d0cb1b954.

Differential Revision: https://code.wildfiregames.com/D2304
Tested on: clang 8.0.1., VS2015, Jenkins
Comments By: Vladislav, Angen, Stan
Tested by: Angen
This was SVN commit r22941.

source/gui/CGUIString.cpp

@@ -21,7 +21,6 @@
 
 #include "graphics/FontMetrics.h"
 #include "gui/CGUI.h"
-#include "gui/GUI.h"
 #include "lib/utf8.h"
 #include "ps/CLogger.h"