Change tui_show_symtab_source to be a method

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.