Install ptf afpacket module required by ptf_nn_agent. (#14503)

Why I did it
ptf_nn_agent failed to start in dnx rpc syncd because module afpacket was not installed.
Please see issue sonic-net/sonic-mgmt#7822

How I did it
Add downloading ptf afpacket module in docker file.

How to verify it
Verified that ptf_nn_agent was started successfully in dnx rpc syncd with the change.

platform/broadcom/docker-syncd-brcm-dnx-rpc/Dockerfile.j2

@@ -49,6 +49,7 @@ RUN wget https://github.com/nanomsg/nanomsg/archive/1.0.0.tar.gz \
  && mkdir -p /opt       \
  && cd /opt             \
  && wget https://raw.githubusercontent.com/p4lang/ptf/master/ptf_nn/ptf_nn_agent.py \
+ && mkdir ptf && cd ptf && wget https://raw.githubusercontent.com/p4lang/ptf/master/src/ptf/afpacket.py && touch __init__.py \
  && apt-get clean -y; apt-get autoclean -y; apt-get autoremove -y \
  && rm -rf /root/deps