ui/console: make qemu_console_is_multihead() static

qemu_console_is_multihead() is only called from within "ui/console.c";
make it static.

Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com> (odd fixer:Graphics)
Cc: Gerd Hoffmann <kraxel@redhat.com> (odd fixer:Graphics)
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20230913144959.41891-2-lersek@redhat.com>