Include llvm linker

Install the LLVM linker; at least one test configuration requires it,
i.e fvp-default-clang,nil,nil,nil:nil, otherwise we see this log on
the build phase

    Makefile:237: *** "No ld.lld in PATH, make sure it is installed or set LD to a different linker".  Stop.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I86e86ec14795ac83a212c92b58bdff21e0f0a9f1
diff --git a/bionic-amd64-tf-a-build/Dockerfile b/bionic-amd64-tf-a-build/Dockerfile
index ad24cc7..fc067fe 100644
--- a/bionic-amd64-tf-a-build/Dockerfile
+++ b/bionic-amd64-tf-a-build/Dockerfile
@@ -26,6 +26,7 @@
     libffi-dev \
     libssl-dev \
     libxml-libxml-perl \
+    lld \
     openjdk-8-jdk \
     openssh-server \
     perl \