rpcserver: fix SignCompact

[yyforyongyu]

Introduced in #2211, we need to update the rpcserver.go to use this method. In addition we need to tag a new version for btcec. We also need to update lnd when the version is tagged. Will update the relevant packages once it's tagged.

[Roasbeef]

⛰   git push origin btcec/v2.3.4
Enumerating objects: 1, done.
Counting objects: 100% (1/1), done.
Writing objects: 100% (1/1), 1.38 KiB | 707.00 KiB/s, done.
Total 1 (delta 0), reused 0 (delta 0), pack-reused 0
To github.com:btcsuite/btcd.git
 * [new tag]             btcec/v2.3.4 -> btcec/v2.3.4
 * ```

[guggero]

Oops, I didn't see there's already a PR for it... Created a new one here: #2218

[yyforyongyu]

Replaced by #2218