add libconfig++-dev package

phosphor-pid-control presently uses libconfig++, therefore
it's required to be pulled into the CI to verify builds.

Change-Id: I775b7cc2d0d82ddd25e22f9f31934d1b37c9a390
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 7850c06..42a227d 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -82,7 +82,8 @@
     clang-format-5.0 \
     iproute2 \
     libnl-3-dev \
-    libnl-genl-3-dev
+    libnl-genl-3-dev \
+    libconfig++-dev
 
 RUN pip install inflection
 RUN pip install pycodestyle