Add placeholder diskattacher for other OSes.

builder/azure/chroot/diskattacher_other.go

@@ -0,0 +1,11 @@
+// +build !linux,!freebsd
+
+package chroot
+
+import (
+	"context"
+)
+
+func (da diskAttacher) WaitForDevice(ctx context.Context, lun int32) (device string, err error) {
+	panic("The azure-chroot builder does not work on this platform.")
+}