[Gui] So headers: remove some unused includes

- also sort out some headers for precompiled headers

src/Gui/SoFCBackgroundGradient.cpp

@@ -20,11 +20,19 @@
  *                                                                         *
  ***************************************************************************/
 
-
 #include "PreCompiled.h"
 
+#ifndef _PreComp_
+#ifdef FC_OS_MACOSX
+#include <OpenGL/gl.h>
+#else
+#include <GL/gl.h>
+#endif
+#endif
+
 #include "SoFCBackgroundGradient.h"
 
+
 using namespace Gui;
 
 SO_NODE_SOURCE(SoFCBackgroundGradient)