Add libsnmp-dev

Phosphor-snmp uses net-snmp,therefore it is required
to pull this library into the CI to verify builds.

Change-Id: Id619cfb3a6e9505eb31329eccaed53d0e07ae656
Signed-off-by: Ratan Gupta <ratagupt@in.ibm.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 42a227d..9af0c5a 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -83,7 +83,8 @@
     iproute2 \
     libnl-3-dev \
     libnl-genl-3-dev \
-    libconfig++-dev
+    libconfig++-dev \
+    libsnmp-dev
 
 RUN pip install inflection
 RUN pip install pycodestyle