[FEM] revert incorrect header removal

- also add comment to prevent this in future

src/Mod/Fem/FemGlobal.h

@@ -20,6 +20,11 @@
  *                                                                         *
  ***************************************************************************/
 
+ // this file contains the export macros for the App and Gui module
+ // for Qt Designer, see e.g. 
+ // https://forum.freecadweb.org/viewtopic.php?p=558050#p558050
+ // why this is necessary
+
 #include <FCGlobal.h>
 
 #ifndef FEM_GLOBAL_H

src/Mod/Fem/Gui/ViewProviderFemMesh.h

@@ -26,6 +26,7 @@
 #include <Gui/ViewProviderBuilder.h>
 #include <Gui/ViewProviderGeometryObject.h>
 #include <Gui/ViewProviderPythonFeature.h>
+#include <Mod/Fem/FemGlobal.h>
 
 class SoCoordinate3;
 class SoDrawStyle;

src/Mod/Fem/Gui/ViewProviderFemPostObject.h

@@ -25,6 +25,7 @@
 
 #include <Base/Observer.h>
 #include <Gui/ViewProviderGeometryObject.h>
+#include <Mod/Fem/FemGlobal.h>
 
 #include <vtkAppendPolyData.h>
 #include <vtkExtractEdges.h>