Add python-protobuf for nanopb

Nanopb depends on python-protobuf to generate codes.

Tested locally by building the image and codes depending on
nanopb.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Iaaf9cb696a27c4c266d66b9d26ee7e281ebaf810
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index bea8030..3cbc23c 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -273,6 +273,7 @@
 RUN pip3 install pycodestyle
 RUN pip3 install jsonschema
 RUN pip3 install meson==0.54.3
+RUN pip3 install protobuf
 
 FROM openbmc-base as openbmc-lcov
 RUN curl -L https://github.com/linux-test-project/lcov/archive/${PKG_REV['lcov']}.tar.gz | tar -xz && \