Install PyYAML in Fuchsia Docker container (llvm#262)

This is needed by LLVM libc newhdrgen.

Author: petrhosek

buildbot/fuchsia/Dockerfile

@@ -18,6 +18,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     python3 \
     python3-pip \
     python3-psutil \
+    python3-yaml \
     software-properties-common \
     unzip \
     # Clean apt cache to reduce image size.