Add systemd-python to optional dependencies (#4339)

Using systemd-python allows for logging to the systemd journal,
as is documented in: `synapse/contrib/systemd/log_config.yaml`.

Signed-off-by: Silke Hofstra <silke@slxh.eu>

docker/Dockerfile-dhvirtualenv

@@ -50,7 +50,9 @@ RUN apt-get update -qq -o Acquire::Languages=none \
         debhelper \
         devscripts \
         dh-systemd \
+        libsystemd-dev \
         lsb-release \
+        pkg-config \
         python3-dev \
         python3-pip \
         python3-setuptools \