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: NDavid S. Miller <davem@davemloft.net>

net/core/stream.c

@@ -43,7 +43,6 @@ void sk_stream_write_space(struct sock *sk)
 		rcu_read_unlock();
 	}
 }
-EXPORT_SYMBOL(sk_stream_write_space);
 
 /**
  * sk_stream_wait_connect - Wait for a socket to get into the connected state