slider: fixed build warning
slider: fixed build warning

Summary: added EINA_UNUSED to fix build warning

Test Plan: N/A

Reviewers: raster, seoz, Hermet

Reviewed By: seoz

Differential Revision: https://phab.enlightenment.org/D777

tasn/elementary,tasn/elementary,tasn/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,rvandegrift/elementary,rvandegrift/elementary,FlorentRevest/Elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary