bug#20506: 25.0.50; elisp-xref-find fails to find system-name.

[Helmut Eller]

Currently this test fails:

(require 'ert)
(require 'cl-lib)
(require 'xref)
(require 'elisp-mode)

(ert-deftest xref-tests-elisp ()
  "Test xref-elisp backend."
  ;; "system-name" is both: a variable and a function
  (should (= (length (elisp-xref-find 'definitions "system-name")) 2)))