unix: fix build on solaris after CL 259637

The build on illumos and solaris currently fails with:

  unix/syscall_solaris.go:558:6: no new variables on left side of :=

Fix this by dropping the named return value.

Change-Id: Ie29956ba08fd107f380fe432cc85f120d66057b5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/259581
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/syscall_solaris.go b/unix/syscall_solaris.go
index fcc0e10..fee6e99 100644
--- a/unix/syscall_solaris.go
+++ b/unix/syscall_solaris.go
@@ -554,7 +554,7 @@
 
 //sys	ioctl(fd int, req uint, arg uintptr) (err error)
 
-func IoctlSetTermio(fd int, req uint, value *Termio) (err error) {
+func IoctlSetTermio(fd int, req uint, value *Termio) error {
 	err := ioctl(fd, req, uintptr(unsafe.Pointer(value)))
 	runtime.KeepAlive(value)
 	return err