This is necessary to make sure that start_line is updated after a
command has been entered.  Usually, start_line gets updated anyway
because most commands output text, and outputting text is done through
the function tui_puts, which updates start_line.  However if a command
does not output text, then tui_puts will not get called and start_line
will not get updated in time for the next prompt to be displayed.

One can observe this bug by executing the command "delete" within TUI.
After executing, the prompt line

  (gdb) delete

gets overwritten by the next prompt.  With this patch, the prompt line
gets preserved.

gdb/ChangeLog:

	* tui/tui-io.c (tui_getc): Use tui_putc instead of waddch to
	emit the newline.