Ricardo found a problem while working on some emacs package that
requires EMMS: the elisp file(s) of this package is(are) not compiled.

This happens because currently emacs-build-system adds only
"share/emacs/site-lisp/guix.d/<package>" to emacs load-path when it
compiles elisp files, but some emacs packages (for example, emms) put
their files in "share/emacs/site-lisp", so this directory should also be
added.  The attached patch will do it.

(I added a copyright line for David because he forgot to do it in commit 578b96af69057883a2a49a34dd6fe261cb2f4e5c)