Add libpthreadpool to development Docker image

After the bump to a more recent tflite version, a new system dependency,
libpthreadpool (under the libpthreadpool-dev package on debian) was
introduced. I forgot to add this dependency to the development docker
image, so it currently fails to build. This patch fixes that.

experimental/docker/development.Dockerfile

@@ -34,7 +34,8 @@ RUN apt-get update && \
     software-properties-common \
     cmake \
     git \
-    vim
+    vim \
+    libpthreadpool-dev
 RUN mkdir /tflite
 WORKDIR /tflite
 COPY buildbot/build_tflite.sh ./