lnd: plug in graph SQL migration into dev build

This commit plugs in the graph kvdb-to-sql migration for builds
containing the `test_native_sql` tag. This will allow us to perform
local tests and write itests for the migration without exposing it to
the production release build.

sqldb/migrations_dev.go

@@ -8,4 +8,13 @@ var migrationAdditions = []MigrationConfig{
 		Version:       8,
 		SchemaVersion: 7,
 	},
+	{
+		Name:          "kv_graph_migration",
+		Version:       9,
+		SchemaVersion: 7,
+		// A migration function may be attached to this
+		// migration to migrate KV graph to the native SQL
+		// schema. This is optional and can be disabled by the
+		// user if necessary.
+	},
 }