Update Test.hs

I was trying to build this with with ghc-head and got the following complaint. Things are fine if I add TypeFamilies, but it seemed better just to add a clarifying signature `addVec :: Word64 -> Int -> IO ()`

    Crypto/Random/Test.hs:53:9:
    Illegal equational constraint primitive-0.5.3.0:Control.Monad.Primitive.PrimState
                                    m
                                  ~ ghc-prim:GHC.Prim.RealWorld
    (Use GADTs or TypeFamilies to permit this)
    When checking that ‘addVec’ has the inferred type
      addVec :: forall (m :: * -> *).
                (primitive-0.5.3.0:Control.Monad.Primitive.PrimMonad m,
                 primitive-0.5.3.0:Control.Monad.Primitive.PrimState m
                 ~ ghc-prim:GHC.Prim.RealWorld) =>
                Word64 -> Int -> m ()
    In an equation for ‘randomTestAppend’:
        randomTestAppend (RandomTestState buckets)
          = loop
          where
              loop bs
                | L.null bs = return ()
                | otherwise
                = do { let ...;
                       .... }
              addVec a i = M.read buckets i >>= \ d -> M.write buckets i $! d + a