explicitly close the DB during importdb

There is no change in behavior since committing the transaction
already write(2)'s all the data to disk. But let's comply with
the official buntdb API.

irc/import.go

@@ -242,5 +242,7 @@ func ImportDB(config *Config, infile string) (err error) {
 		return doImportDB(config, dbImport, tx)
 	}
 
-	return db.Update(performImport)
+	err = db.Update(performImport)
+	db.Close()
+	return
 }