gdp: move dataprotocol library into gdp plugin and make private

gdp: move dataprotocol library into gdp plugin and make private

We have removed things like protocol=gdp in the tcp elements
in favour of explicit gdppay/depay elements, so there's no need
to keep a public API and library for now. We can still add it
back later. Someone needs to think hard about 0.11 and gdp
anyway one of these days.