add missing deprecation require

[nerdrew]

I get an error when requiring blah.pb.rb files without this fix

protobuf/field/base_field.rb:156:in `<class:BaseField>': undefined method `deprecator' for Protobuf:Module (NoMethodError)
protobuf/field/base_field.rb:7:in `<module:Field>'
protobuf/field/base_field.rb:6:in `<module:Protobuf>'
protobuf/field/base_field.rb:5:in `<top (required)>'
protobuf/field.rb:1:in `require'
protobuf/field.rb:1:in `<top (required)>'
protobuf/message.rb:1:in `require'
protobuf/message.rb:1:in `<top (required)>'

[zachmargolis]

LGTM