Allow to add irt symbols with nacl-irt command.

gdb/nacl-manifest.h

 /* Handle Native Client manifest files.
 
    Copyright (C) 2011 Free Software Foundation, Inc.
 
    This file is part of GDB.
 
    This program 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.
 
    This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #ifndef NACL_MANIFEST_H
 #define NACL_MANIFEST_H
 
 /* Real filename for Native Client program.  */
 const char *nacl_manifest_program (void);
 
-/* Real filename for Native Client IRT.  */
-const char *nacl_manifest_irt (void);
-
 /* Real filename for Native Client original_name.  */
 const char *nacl_manifest_find (char *original_name);
 
 #endif