eina_cxx: Fix eina::value copy constructor.

eina_cxx: Fix eina::value copy constructor.

Summary:
The previous declaration was causing the implicit copy constructor to be
invoked when copying a new eina::value from an existing eina::value,
thus leading to two wrappers pointing to the same underlying C pointer.

This showed some error messages when running the tests but under autotools
it didn't fail, while building with meson caused it to segfault.

Reviewers: vitor.sousa

Reviewed By: vitor.sousa

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D7166