eina_cxx: Fix eina::value copy constructor.
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.
Reviewed By: vitor.sousa
Differential Revision: https://phab.enlightenment.org/D7166