gnu: emacs-helm: Update to 3.6.4.

From 451290515c14b44aa00bd542e13a00a3898d4a90 Mon Sep 17 00:00:00 2001
From: Michael Rohleder <mike@rohleder.de>
Date: Mon, 20 Jul 2020 18:36:09 +0200
Subject: [PATCH] gnu: emacs-helm: Update to 3.6.4.

* gnu/packages/emacs-xyz.scm (emacs-helm): Update to 3.6.4.

Signed-off-by: Oleg Pykhalov <go.wigust@gmail.com>