Exclude pg_query.pb-c.o from package

[seanlinsley]

Followup to #53

This is listed in the libpg_query gitignore file, but was accidentally included in the cargo package, resulting in this error:

cargo package
error: 1 files in the working directory contain changes that were not yet committed into git:

libpg_query/protobuf/pg_query.pb-c.o

to proceed despite this and include the uncommitted changes, pass the `--allow-dirty` flag