Add default value for MakeConfigFileName() 2nd argument

Make it possible to call wxStandardPaths::MakeConfigFileName() without
explicitly specifying the convention to use, to make it conform to the
documentation (previously this only worked when calling the
wxStandardPathsBase version of the method).

Closes #18106.

include/wx/unix/stdpaths.h

@@ -50,7 +50,8 @@ public:
     virtual wxString GetUserDir(Dir userDir) const wxOVERRIDE;
 #endif
     virtual wxString MakeConfigFileName(const wxString& basename,
-                                        ConfigFileConv conv) const wxOVERRIDE;
+                                        ConfigFileConv conv = ConfigFileConv_Ext
+                                        ) const wxOVERRIDE;
 
 protected:
     // Ctor is protected, use wxStandardPaths::Get() instead of instantiating