symtab.c: Small refactor
    
    add_symtab_completions does the exact same as the code that it is
    replacing.
    
    gdb/ChangeLog:
    2017-02-24  Pedro Alves  <palves@redhat.com>
    
    	* symtab.c (make_file_symbol_completion_list_1): Use
    	add_symtab_completions.