MAINTAINERS: Add include/sysemu/qtest.h to the qtest section

We already list system/qtest.c in the qtest section, so the
corresponding header file should be listed here, too.

Message-ID: <20231012111401.871711-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>