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

lily/melody-spanner.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 2005--2020 Han-Wen Nienhuys <hanwen@xs4all.nl>
 
 
   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/>.
 */
 
 #include "melody-spanner.hh"
 #include "grob.hh"
 #include "pointer-group-interface.hh"
 
+using std::vector;
+
 /*
   TODO: this could be either item or spanner. For efficiency reasons,
   let's take item for now.
 */
 
 /*
   Interpolate stem directions for neutral stems.
  */
 MAKE_SCHEME_CALLBACK (Melody_spanner, calc_neutral_stem_direction, 1);
 SCM
@@ skipping 68 matching lines @@
 }
 
 ADD_INTERFACE (Melody_spanner,
                "Context dependent typesetting decisions.",
 
                /* properties */
                "stems "
                "neutral-direction "
               );