gnu: emacs: Fix ert-runner by adding 'reporters' subdirectory.

Previous this change, ert-runner would fail with error:
"Invalid reporter: dot".

* gnu/packages/emacs.scm (ert-runner)[include]: Add regexp to match elisp
files under the 'reporters' subdirectory.

Signed-off-by: Arun Isaac <arunisaac@systemreboot.net>