unix: add Msghdr.SetIovlen for solaris/amd64

Follow-up for CL 194077

Updates golang/go#34164

Change-Id: I4bf670dc3e13c483e9087dc3f04973278a09b2f4
Reviewed-on: https://go-review.googlesource.com/c/sys/+/194099
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_solaris_amd64.go

@@ -18,6 +18,10 @@ func (iov *Iovec) SetLen(length int) {
 	iov.Len = uint64(length)
 }
 
+func (msghdr *Msghdr) SetIovlen(length int) {
+	msghdr.Iovlen = int32(length)
+}
+
 func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }