gnu: Add missing references to (gnu packages cran).

This is a follow-up to commit a8cba9dd11.

Reported-by: Efraim Flashner <efraim@flashner.co.il>

gnu/packages/machine-learning.scm

@@ -32,6 +32,7 @@
   #:use-module (gnu packages autotools)
   #:use-module (gnu packages boost)
   #:use-module (gnu packages compression)
+  #:use-module (gnu packages cran)
   #:use-module (gnu packages dejagnu)
   #:use-module (gnu packages gcc)
   #:use-module (gnu packages image)