Dockerfile: add graphviz package

If one wants to use the docker image for building Zephyr's Doxygen
documnentation (which the project's CI hasn't actually done in a while
since docs are built using GH runners these days), graphviz (`dot`) is
needed.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

Dockerfile.base

@@ -96,6 +96,7 @@ RUN <<EOF
 		git-core \
 		gnupg \
 		gperf \
+		graphviz \
 		help2man \
 		iproute2 \
 		lcov \