xenial-amd64-mbed-tls-build: install the 'lsb' package

Needed for basic-build-test.sh script

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: Ia30f56fa8054991a50b0bd43647fb50889cb9cef
diff --git a/xenial-amd64-mbed-tls-build/Dockerfile b/xenial-amd64-mbed-tls-build/Dockerfile
index 89c4782..f9ffc6a 100644
--- a/xenial-amd64-mbed-tls-build/Dockerfile
+++ b/xenial-amd64-mbed-tls-build/Dockerfile
@@ -69,6 +69,8 @@
         libtasn1-6-dev \
         # needed for armcc (see locale-gen below)
         locales \
+        # needed for basic-build-test.sh
+        lsb \
         # used by compat.sh and ssl-opt.sh
         lsof \
         # to build GnuTLS (nettle)