Only declare wxTextCtrl::MSWCreateContextMenu() if wxUSE_RICHEDIT==1

This method is only used, and defined, when using rich edit controls, so don't
declare it if wxUSE_RICHEDIT==0.

This should fix a link error in this build variant.

Closes https://github.com/wxWidgets/wxWidgets/pull/452

include/wx/msw/textctrl.h

@@ -171,10 +171,12 @@ public:
     // EDIT control has one already)
     void OnContextMenu(wxContextMenuEvent& event);
 
+#if wxUSE_RICHEDIT
     // Create context menu for RICHEDIT controls. This may be called once during
     // the control's lifetime or every time the menu is shown, depending on
     // implementation.
     virtual wxMenu *MSWCreateContextMenu();
+#endif // wxUSE_RICHEDIT
 
     // be sure the caret remains invisible if the user
     // called HideNativeCaret() before