tcp: Unexport TCPv6 GRO functions

Sinec the TCPv6 GRO functions are used in the same file where
they are defined, we do not need to export them.  This was a
cut-n-paste from the IPv4 code which does need to export them.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>