Now that model.h is in the repo, it no longer is a build dependency.
This allows compilation of the test without having emlearn installed,
which is useful e.g. for build testing.
The Makefile rule to regenerate model.h has been left in place, but
just adding the header to the test allows to build the application
without heaving emlearn installed, which is convenient for the CI.
Actual users of emlearn will like still want to generate the header.
