unix: use keyed composite literal in TestSelect

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>

unix/syscall_linux_test.go

@@ -178,7 +178,7 @@ func TestGetrlimit(t *testing.T) {
 }
 
 func TestSelect(t *testing.T) {
-	_, err := unix.Select(0, nil, nil, nil, &unix.Timeval{0, 0})
+	_, err := unix.Select(0, nil, nil, nil, &unix.Timeval{Sec: 0, Usec: 0})
 	if err != nil {
 		t.Fatalf("Select: %v", err)
 	}