Dockerfile: Install pkgconfig

We missed updating this list of build dependencies when we began to use
`pkgconfig`.

Signed-off-by: Matt Wozniski <godlygeek@gmail.com>

Dockerfile

@@ -45,6 +45,7 @@ RUN $PYTHON -m pip install -U \
     -r /tmp/requirements-test.txt \
     -r /tmp/requirements-docs.txt \
     cython \
+    pkgconfig \
     setuptools \
     wheel