Handle signedness of struct members

struct members are currently always treated as unsigned. This leads to
incorrect behavior in contexts where the sign makes a difference. E.g.

```
struct packed {
  int x;
} s;

s.x = -1;
if (s.x > 0) $display("FAILED");
```

Query the signedness of the struct member form the type of the member.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

netstruct.h

@@ -34,8 +34,8 @@ class netstruct_t : public LineInfo, public ivl_type_s {
 	    ivl_type_t net_type;
 	    inline ivl_variable_type_t data_type() const
 	    { return net_type->base_type(); };
-	      // We need to keep the individual element sign information.
-	    bool get_signed() const { return false; };
+	    bool get_signed() const
+	    { return net_type->get_signed(); }
       };
 
     public: