Include libxml-libxml-perl package

The libxml package is required to build TF-A tests as indicated in
[1] otherwise build failures are encountered when building tftf
related test configs.

[1] https://trustedfirmware-a-tests.readthedocs.io/en/latest/getting_started/requirements.html#build-host

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I0c22a219f27a5164037764604cbf23d7790c1e88
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index edcd07e..7376014 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -23,6 +23,7 @@
     lcov \
     libffi-dev \
     libssl-dev \
+    libxml-libxml-perl \
     openjdk-8-jdk \
     openssh-server \
     perl \