Fixes the following vet error:

  syscall_linux_test.go:181: golang.org/x/sys/unix.Timeval composite literal uses unkeyed fields

Change-Id: I6973ad4ecd54b542074610a755573f98eb23f343
Reviewed-on: https://go-review.googlesource.com/75790
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>