tests/docker: Install liblttng-ust-dev package in Ubuntu 20.04 image

tests/docker: Install liblttng-ust-dev package in Ubuntu 20.04 image

Install the liblttng-ust-dev package to be able to
build QEMU using the User-Space Tracer trace backend
(configure --enable-trace-backends=ust).

Suggested-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20201111121234.3246812-2-philmd@redhat.com>
Message-Id: <20201117173635.29101-6-alex.bennee@linaro.org>