gnu: chemistry.scm: Add missing import.

This is a follow-up to commit eb5ece73a8.

* gnu/packages/chemistry.scm: Import (gnu packages).

gnu/packages/chemistry.scm

@@ -21,6 +21,7 @@
   #:use-module (guix packages)
   #:use-module ((guix licenses) #:prefix license:)
   #:use-module (guix download)
+  #:use-module (gnu packages)
   #:use-module (gnu packages algebra)
   #:use-module (gnu packages compression)
   #:use-module (gnu packages gv)