slider: fix edje warning

slider: fix edje warning

Summary:
ERR<18878>:edje lib/edje/edje_calc.c:2767 _edje_part_recalc_single() file /usr/local/share/elementary/themes/default.edj, group elm/slider/horizontal/default has a non-fixed part 'elm.swallow.icon'. You should add 'fixed: 1 0'. But in order to optimize the edje calc, we add it automatically.
ERR<18878>:edje lib/edje/edje_calc.c:2767 _edje_part_recalc_single() file /usr/local/share/elementary/themes/default.edj, group elm/slider/horizontal/default has a non-fixed part 'elm.swallow.end'. You should add 'fixed: 1 0'. But in order to optimize the edje calc, we add it automatically.

Test Plan: Launch an app with slider, open the slider and observe the console output

Reviewers: cedric, singh.amitesh, jpeg

Subscribers: seoz, minkyu

Differential Revision: https://phab.enlightenment.org/D4117