python-jinja2: Disable `make clean`

Makefile doesn't define "clean". Unless disabled, base_do_configure()
will fail the build attempting to run `make clean` when rebuilding
this recipe.

Signed-off-by: Haris Okanovic <haris.okanovic@ni.com>
Natinst-ReviewBoard-ID: 119389
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>