build: enable namespace verification for default theme

Summary:
this will trigger a build failure if someone modifies the theme in such
a way that namespacing is not used correctly, saving some time when
reviewing larger patches which have many theme changes

ref T6911
Depends on D6036

Reviewers: cedric

Reviewed By: cedric

Subscribers: committers

Tags: efl

Maniphest Tasks: T6911

Differential Revision: https://phab.enlightenment.org/D6042