tcp: export timestamp helpers for mptcp

MPTCP is builtin, so no need to add EXPORT_SYMBOL()s.

It will be used to support SO_TIMESTAMP(NS) ancillary
messages in the mptcp receive path.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>