Fix pygit2-libgit2 builds on Debian like platforms.

Signed-off-by: Rene Jochum <rene@jochums.at>

salt/gitfs/pygit2.sls

@@ -23,6 +23,7 @@ pygit-deps:
   pkg.installed:
     - pkgs:
       - build-essential
+      - pkg-config
       - python-dev
       - libssh-dev
       - libffi-dev