unix: add SizeofIovec on aix, dragonfly, *bsd and solaris

Change-Id: Ib981ad3f745a2566d0a12eba5e54c7da53b69d6e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/279112
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
diff --git a/unix/ztypes_freebsd_arm.go b/unix/ztypes_freebsd_arm.go
index b91c2ae..5da13c8 100644
--- a/unix/ztypes_freebsd_arm.go
+++ b/unix/ztypes_freebsd_arm.go
@@ -311,6 +311,7 @@
 	SizeofSockaddrUnix     = 0x6a
 	SizeofSockaddrDatalink = 0x36
 	SizeofLinger           = 0x8
+	SizeofIovec            = 0x8
 	SizeofIPMreq           = 0x8
 	SizeofIPMreqn          = 0xc
 	SizeofIPv6Mreq         = 0x14