windows: fix go vet warning
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/75973
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys