Avoid string_file in tui_make_status_line
    
    tui_make_status_line uses string_file where a simple std::string
    constructor would do.  This makes this change.
    
    gdb/ChangeLog
    2019-08-15  Tom Tromey  <tom@tromey.com>
    
    	* tui/tui-stack.c (tui_make_status_line): Use string constructor.