nanopb.proto: add back in again

This broke external (i.e. non-machinekit builds) like QtQuickVCP

we'll keep nanopb.proto in machinetalk-protobuf and check during
the machinekit build if the nanopb.proto file from this repo differs
from the one which is brought into the machinekit repo as a git subtree
from machinekit/nanopb (pathname src/machinetalk/nanopb/generator/proto/nanopb.proto)

Since the change frequency of nanopb.proto is fairly low and external builds
are unlikely to use the nanopb options, using an outdated nanopb.proto
in an external build is unlikely to cause issues