soc2c: the c code was wrong in presence of user type defined using extern types.

The previous change fixed program that contains type decl such as :
type t1;
type t2 = t1;

but not

type t1;
type t2 = t1^2 ;

or

type t2 = { xxx : t1^2 ; yyy : t1^2} ;

And now it works.

To fix it, I have include the _ext.h file into the lustre_type.h one.