Sync dependencies

pybind11 is not used in any project yet, but we have to bring it in via
Zuul because it's a (transitive) git submodule. Let's pin the SCM
version for a future Buildroot package as well while we're at this.

Change-Id: I0b09ca4c9d60e95c7d1a62c1f11f7e7a53bf2841
diff --git a/.zuul.yaml b/.zuul.yaml
index 5b42586..a1c97b3 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -21,6 +21,7 @@
       - github/rollbear/trompeloeil
       - github/tplgy/cppcodec
       - github/AmokHuginnsson/replxx
+      - github/pybind/pybind11
 
 - job:
     name: czechlight-beagleboneblack