python: add __init__.py in protobuf directory

Once we have setup.py for our python scripts, that __init__.py will be
required to include this directory in the resulting archive.

Also, changed .gitignore order to make sure that __init__.py isn't
ignored.

Signed-off-by: Konrad Sztyber <konrad.sztyber@intel.com>
Change-Id: Ie00e04e131059c8c968983ab44249ca6b6a296f7
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/17934


Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Michal Berger <michal.berger@intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>