dfmc-llvm-linker: Fix raw type references

This fixes a problem caused by references to FFI struct types.

* sources/dfmc/llvm-linker/llvm-link-object.dylan
  (emit-extern on <&raw-aggregate-type>): Don't emit anything for
   extern references to virtual objects such as FFI struct types.

sources/dfmc/llvm-linker/llvm-link-object.dylan

@@ -311,3 +311,8 @@ define method emit-indirection-definition
   llvm-builder-define-global(back-end, name, global);
 end method;
 
+define method emit-extern
+    (back-end :: <llvm-back-end>, module :: <llvm-module>,
+     object :: <&raw-aggregate-type>) => ()
+  // These are virtual objects, no need to emit them
+end;