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>