elm_box.eo: Added @ingroup tag.
elm_box.eo: Added @ingroup tag.

Summary: The API "elm_box_horizontal_get" missed @ingroup tag. so i added the tag for grouping when  doxygen making API documentation.

Reviewers: Hermet, JackDanielZ, Jaehyun, seoz

Reviewed By: seoz

Differential Revision: https://phab.enlightenment.org/D1966

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