Fix placement of output in TUI mode

Fix placement of output in TUI mode

The fix for PR tui/28819 regressed gdb command output a bit.  In
"nonl" mode, pressing the Enter key will result in a newline not being
echoed properly, so that gdb output for the command will begin on the
same line as the input.

This patch changes gdb_wgetch to echo the newline.  I have only tested
this interactively, as the TUI doesn't have automated tests in
general.

gdb/ChangeLog
2019-01-14  Tom Tromey  <tom@tromey.com>

	PR tui/28819:
	* tui/tui-io.c (gdb_wgetch): Print \r when needed.