[crypto/openpgp] Serialize encrypted PrivateKey

[tcz001]

Change-Id: Ieedafd9098f8ed20de26725cb58ea98ef0823f61

[gopherbot]

Hi! Thanks for the PR!

Unfortunately, the Go project doesn't use GitHub's Pull Requests,
so we can't accept your contribution this way.
We instead use a code review system called Gerrit.

The good news is, I'm here to help.

From here, you have two options:

1. Read our Contribution Guidelines to learn how to send a change with Gerrit.
2. Or, create an issue about the issue this PR addresses, so that someone else can fix it.

I'm going to close this Pull Request now.
Please don't be offended! :-)

Thanks again,

GopherBot (on behalf of the Go Team)