net: do not export sk_stream_write_space

Since commit 900f65d3 ("tcp: move duplicate code from
tcp_v4_init_sock()/tcp_v6_init_sock()") we no longer need
to export sk_stream_write_space()

From: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>

Signed-off-by: David S. Miller <davem@davemloft.net>