Remove leftover #endif after introducing SO_REUSEPORT

Commit 055dc21a (soreuseport: infrastructure) removed the #if 0
around SO_REUSEPORT without removing the corresponding #endif
thus causing the header guard to close early.

Cc: Tom Herbert <therbert@google.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>