Fix dhPubEq

[bjin]

I actually don't understand the root cause, how type families allows Eq instance to be derived without explicit deriving statement. But with ghc 8.0.2 and cacophony 0.9.2, the following code would reach bottom (infinite loop).

import Crypto.Noise.DH
import Crypto.Noise.DH.Curve25519

main :: IO ()
main = do
    key1 <- snd <$> dhGenKey :: IO (PublicKey Curve25519)
    key2 <- snd <$> dhGenKey :: IO (PublicKey Curve25519)
    print (key1 == key2)

[centromere]

Thank you!