Turn tui_show_exec_info_content into a method

Turn tui_show_exec_info_content into a method

This changes tui_show_exec_info_content to be a method on
tui_source_window_base.  As it is only called by other methods on this
class, it can be private.

gdb/ChangeLog
2019-08-13  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_base)
	<show_exec_info_content>: Declare.
	(tui_show_exec_info_content): Don't declare.
	* tui/tui-winsource.c
	(tui_source_window_base::show_exec_info_content): Rename from
	tui_show_exec_info_content.
	(tui_source_window_base::update_exec_info): Update.