Provide default value for wxMkDir() "mode" under all platforms

Previously this function could be called with just path only under
Windows and not when using Cygwin. Make it possible to always omit mode
and use the appropriate default value in this case.

This improves consistency and fixes build of wxWidgets itself under
Cygwin.

Closes #26042.

include/wx/filefn.h

@@ -381,7 +381,7 @@ inline int wxRmDir(const wxString& path)
 inline int wxMkDir(const wxString& path, mode_t WXUNUSED(mode) = 0)
     { return wxCRT_MkDir(path.fn_str()); }
 #else
-inline int wxMkDir(const wxString& path, mode_t mode)
+inline int wxMkDir(const wxString& path, mode_t mode = 0777)
     { return wxCRT_MkDir(path.fn_str(), mode); }
 #endif