Change tui_show_symtab_source to be a method
    
    This changes tui_show_symtab_source to be a method on
    tui_source_window.
    
    gdb/ChangeLog
    2019-08-16  Tom Tromey  <tom@tromey.com>
    
    	* tui/tui-winsource.c (tui_update_source_windows_with_addr)
    	(tui_update_source_windows_with_line): Update.
    	* tui/tui-source.h (struct tui_source_window)
    	<show_symtab_source>: Declare.
    	(tui_show_symtab_source): Don't declare.
    	* tui/tui-source.c (tui_show_symtab_source): Rename from
    	tui_show_symtab_source.