protobuf: add license metadata

https://github.com/protocolbuffers/protobuf/blob/master/LICENSE

GitHub doesn't identify it as BSD-3, but it is if you just look at the
license text.

pkgs/development/python-modules/protobuf/default.nix

@@ -59,6 +59,7 @@ buildPythonPackage {
   meta = {
     description = "Protocol Buffers are Google's data interchange format";
     homepage = https://developers.google.com/protocol-buffers/;
+    license = licenses.bsd3;
   };
 
   passthru.protobuf = protobuf;