Change tui_update_source_window to be a method

Change tui_update_source_window to be a method

This changes tui_update_source_window to be a method on
tui_source_window_base.

gdb/ChangeLog
2019-08-16  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_base)
	<update_source_window>: Declare.
	(tui_update_source_window): Don't declare.
	* tui/tui-winsource.c
	(tui_source_window_base::update_source_window): Rename from
	tui_update_source_window.
	(tui_source_window_base::rerender): Update.
	* tui/tui-source.c (tui_source_window::maybe_update): Update.
	* tui/tui-disasm.c (tui_show_disassem)
	(tui_show_disassem_and_update_source)
	(tui_disasm_window::maybe_update): Update.