mod: bump version of kvdb submodule

PR #5992 was merged without a bump in the kvdb version which results in
an error when trying to pull in lnd 0.14.0-beta as a library in other
projects.

We'll need to push the kvdb/v1.2.1 tag _after_ merging this PR.

go.mod

@@ -43,7 +43,7 @@ require (
 	github.com/lightningnetwork/lnd/cert v1.1.0
 	github.com/lightningnetwork/lnd/clock v1.1.0
 	github.com/lightningnetwork/lnd/healthcheck v1.2.0
-	github.com/lightningnetwork/lnd/kvdb v1.2.0
+	github.com/lightningnetwork/lnd/kvdb v1.2.1
 	github.com/lightningnetwork/lnd/queue v1.1.0
 	github.com/lightningnetwork/lnd/ticker v1.1.0
 	github.com/ltcsuite/ltcd v0.0.0-20190101042124-f37f8bf35796