bionic-amd64-tf-a-build: add 'util-linux' package

Updated dockerfile to install 'util-linux' package, needed
by FVP CI scripts to execute 'uuidgen' command.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I066fab1fafaefd1998c260a03c90b8f8ed5eb9d2
diff --git a/bionic-amd64-tf-a-build/Dockerfile b/bionic-amd64-tf-a-build/Dockerfile
index 65f4622..ab7e333 100644
--- a/bionic-amd64-tf-a-build/Dockerfile
+++ b/bionic-amd64-tf-a-build/Dockerfile
@@ -47,6 +47,7 @@
     sudo \
     tree \
     unzip \
+    util-linux \
     virtualenv \
     wget \
     zip \