Fix handling of structs

[ckaestne]

Undefined structs seem to be fine in pointers as long as they are not directly used.

for example

struct x {
  struct y* aa;
} u;

int main() {return 0;}

is well typed, but typechef complains

in contrast the following is not:

struct x {
  struct y aa;
} u;

int main() {return 0;}

[ckaestne]

was not a bug