py: Comment exc_state member from mp_obj_gen_instance_t as it gives trouble

...to some compilers who can't process 2 zero-sized arrays in structs. It's
never referenced directly anyway.

See disussion on #568 as well.

py/objgenerator.c

@@ -90,7 +90,7 @@ typedef struct _mp_obj_gen_instance_t {
     // Variable-length
     mp_obj_t state[0];
     // Variable-length, never accessed by name, only as (void*)(state + n_state)
-    mp_exc_stack_t exc_state[0];
+    //mp_exc_stack_t exc_state[0];
 } mp_obj_gen_instance_t;
 
 void gen_instance_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {