bionic-amd64-tf-m-build: install Ninja

Ninja based build can improve TF-M build job performance.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: Iaabdc549968484dc4d98008899a1a435929a71cf
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index 92100a2..a562fdc 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -37,6 +37,7 @@
     libssl-dev \
     libxml-libxml-perl \
     locales \
+    ninja-build \
     openjdk-8-jdk \
     openssh-server \
     perl \