Efl.Gfx.Vg.Value_Provider: Move internal function changed_flag to eo.

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