Issue 4550: Avoid "using namespace std;" in included files (Take 2)

lily/include/lily-version.hh

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 1999--2020 Jan Nieuwenhuizen <janneke@gnu.org>
 
   LilyPond is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.
 
   LilyPond is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
 
   You should have received a copy of the GNU General Public License
   along with LilyPond.  If not, see <http://www.gnu.org/licenses/>.
 */
 
 #ifndef LILY_VERSION_HH
 #define LILY_VERSION_HH
 
 #include "std-string.hh"
 
-string version_string ();
-string gnu_lilypond_string ();
-string gnu_lilypond_version_string ();
+std::string version_string ();
+std::string gnu_lilypond_string ();
+std::string gnu_lilypond_version_string ();
 
 #endif // LILY_VERSION_HH