Add vendor directory to PKG_CONFIG_PATH
Add vendor directory to PKG_CONFIG_PATH

mfenniak/heroku-buildpack-python-libffi,mfenniak/heroku-buildpack-python-libffi,kennethjiang/heroku-buildpack-python-libffi,kennethjiang/heroku-buildpack-python-libffi,kennethjiang/heroku-buildpack-python-libffi,mfenniak/heroku-buildpack-python-libffi