fix a compile error of r2.1 caused by skylib address invalid.

[rhdong]

fix a compile error of r2.1 caused by the invalid link of skylib.

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

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[gbaned]

@rhdong Can you please sign CLA ? Thanks!

[mihaimaruseac]

Please sign CLA and open against master and then cherry-pick here.

If master already works, we should cherry-pick the commit fixing there.

Note that since this is against 2.1 branch we won't merge it until we need to do a patch release (for security reasons)

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[mihaimaruseac]

We can only merge this when we do a patch release on the 2.1 release