unix: add SizeofIovec on darwin

Change-Id: I50909f3b731b4c0ef7f37788e8132930bb865c2b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/274452
Trust: Tobias Klauser <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matt Layher <[email protected]>

Author: tklauser

unix/types_darwin.go

@@ -178,6 +178,7 @@ const (
 	SizeofSockaddrDatalink = C.sizeof_struct_sockaddr_dl
 	SizeofSockaddrCtl      = C.sizeof_struct_sockaddr_ctl
 	SizeofLinger           = C.sizeof_struct_linger
+	SizeofIovec            = C.sizeof_struct_iovec
 	SizeofIPMreq           = C.sizeof_struct_ip_mreq
 	SizeofIPv6Mreq         = C.sizeof_struct_ipv6_mreq
 	SizeofMsghdr           = C.sizeof_struct_msghdr