build: fixing doc ToX target with missing deps

Author: pthierry-ledger

tox.ini

@@ -52,7 +52,10 @@ commands =
 
 
 [testenv:doc]
-extras = doc
+description = generate sphinx documentation
+extras =
+    doc
+    protoc
 allowlist_externals =
     {toxinidir}/genproto.sh
 commands =