+ fwd decl, made necessary by changed include order

GUI/Support/Tool/Globals.h

@@ -15,6 +15,8 @@
 #ifndef BORNAGAIN_GUI_SUPPORT_TOOL_GLOBALS_H
 #define BORNAGAIN_GUI_SUPPORT_TOOL_GLOBALS_H
 
+class QWidget;
+
 namespace GUI::Global {
 
 inline class QWidget* mainWindow;