Added a missing include (GCC11.1 without PCH).

src/slic3r/GUI/OptionsGroup.cpp

@@ -4,6 +4,7 @@
 #include "GUI_App.hpp"
 #include "OG_CustomCtrl.hpp"
 #include "MsgDialog.hpp"
+#include "format.hpp"
 
 #include <utility>
 #include <wx/numformatter.h>