cmd/vet/all: update windows whitelist

A last-minute rollback of a change left some
unreachable code that we don't want to remove.

Change-Id: Ida0af5b18ed1a2e13ef66c303694afcc49d7bff4
Reviewed-on: https://go-review.googlesource.com/37304
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/vet/all/whitelist/windows.txt

 // windows-specific vet whitelist. See readme.txt for details.
 
-path/filepath/path_windows_test.go: possible formatting directive in Fatal call
+// Issue 18609
+crypto/x509/root_windows.go: unreachable code
+
 runtime/sys_windows_ARCHSUFF.s: [GOARCH] sigtramp: function sigtramp missing Go declaration
 runtime/sys_windows_ARCHSUFF.s: [GOARCH] onosstack: unknown variable usec; offset 0 is fn+0(FP)