fix unused variable error The previous clean up left behind kSocketError which breaks in unittests: shill/connection_health_checker_unittest.cc:62:11: error: unused variable 'kSocketError' [-Werror,-Wunused-const-variable] BUG=chromium:385182 TEST=None Change-Id: I0b6bd48cff80c6f9b3237ea680a85db9d8a44713 Reviewed-on: https://chromium-review.googlesource.com/205360 Reviewed-by: mukesh agrawal <quiche@chromium.org> Tested-by: Mike Frysinger <vapier@chromium.org>