Redraw both spaces between line numbers and source code

There a 2 spaces between the numbers and source code, but only one of
them was redrawn.
So if you increase the source window height, the second space keeps the
character of the border rectangle.

With this both spaces are redrawn, so the border rectangle character is
overwritten.

gdb/ChangeLog:

2021-01-05  Hannes Domani  <ssbssa@yahoo.de>

	* tui/tui-source.c (tui_source_window::show_line_number):
	Redraw second space after line number.