[gdb/tui] Fix secondary prompt

[gdb/tui] Fix secondary prompt

With CLI, a session defining a command looks like:
...
(gdb) define foo
Type commands for definition of "foo".
End with a line saying just "end".
>bar
>end
(gdb)
...

With TUI however, we get the same secondary prompts, and type the same, but
are left with:
...
(gdb) define foo
Type commands for definition of "foo".
End with a line saying just "end".
(gdb)
...

Fix this by calling tui_inject_newline_into_command_window in
gdb_readline_wrapper_line, as is done in tui_command_line_handler.

Tested on x86_64-linux.

PR tui/30636
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30636