Use std::string in macho_symfile_read_all_oso

Use std::string in macho_symfile_read_all_oso

This changes macho_symfile_read_all_oso to use std::string.  This
avoids a cleanup.

2016-10-12  Tom Tromey  <tom@tromey.com>

	* machoread.c (macho_symfile_read_all_oso): Use std::string.