wincred: use go build constraint

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

wincred/cmd/main.go

@@ -1,3 +1,5 @@
+//go:build windows
+
 package main
 
 import (

wincred/wincred.go

@@ -1,3 +1,5 @@
+//go:build windows
+
 package wincred
 
 import (

wincred/wincred_test.go

@@ -1,3 +1,5 @@
+//go:build windows
+
 package wincred
 
 import (