windows: fix go vet warning

Now that go tip runs vet during go test	(CL 74356), the x/sys/windows
test fails due to the following vet error:

  syscall_windows_test.go:84: Fatal call has possible formatting directive %x

Fix it by using Fatalf instead.

Change-Id: I02536312c00bc9002400b29b5b5acef879a0e51a
Reviewed-on: https://go-review.googlesource.com/75973Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/syscall_windows_test.go

@@ -81,7 +81,7 @@ func TestFormatMessage(t *testing.T) {
 	buf := make([]uint16, 300)
 	_, err = windows.FormatMessage(flags, uintptr(dll.Handle), uint32(errno), 0, buf, nil)
 	if err != nil {
-		t.Fatal("FormatMessage for handle=%x and errno=%x failed: %v", dll.Handle, errno, err)
+		t.Fatalf("FormatMessage for handle=%x and errno=%x failed: %v", dll.Handle, errno, err)
 	}
 }