Fix flushing bug in tui_puts_internal

Fix flushing bug in tui_puts_internal

A while back I changed gdb not to flush in some places.  It turned out
that this broke the TUI a little.  An easy way to see it is to run
"gdb -tui -nx", then "file gdb" at the gdb prompt.  gdb will print the
usual "Reading symbols..." message -- but it won't appear on-screen
until the reading is complete.

This patch changes the TUI to do the equivalent of line buffering in
tui_puts_internal.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-io.c (tui_puts_internal): Call wrefresh if newline is
	seen.