Efl.Gfx.Vg.Value_Provider: Move internal function changed_flag to eo.
Summary:
changed_flag was declared in the private header for internal use(static_lib).
However, as the meson script has been changed, it can no longer be used.
changed_flag can display the property to which the changed
among the properties of value_provider.
there is no problem in providing it as an eo function, so move the function to eo.
Test Plan: N/A
Reviewers: Hermet, herb, kimcinoo
Reviewed By: Hermet
Subscribers: cedric, #reviewers, #committers
Tags: #efl
Differential Revision: https://phab.enlightenment.org/D11894