nitx: only depends on modelize_property instead of the full frontend

~~~
$ time ../bin/nitx nitg.nit :q # before
real	0m2.775s
user	0m2.660s
sys	0m0.112s
$ time ./nitx nitg.nit :q # after
real	0m1.436s
user	0m1.328s
sys	0m0.104s
~~~

Signed-off-by: Jean Privat <jean@pryen.org>