Fix GitHub issue #361 - explicit cast check on function return value.

A function call returning an enumeration value can be assigned to an
enumeration variable without an explict cast.

(cherry picked from commit bd0133b386)

netlist.cc

@@ -2154,6 +2154,14 @@ ivl_variable_type_t NetEUFunc::expr_type() const
       return IVL_VT_VOID;
 }
 
+const netenum_t* NetEUFunc::enumeration() const
+{
+      if (result_sig_)
+	    return result_sig_->enumeration();
+
+      return 0;
+}
+
 NetUTask::NetUTask(NetScope*def)
 : task_(def)
 {

netlist.h

@@ -3714,6 +3714,7 @@ class NetEUFunc  : public NetExpr {
       const NetScope* func() const;
 
       virtual ivl_variable_type_t expr_type() const;
+      virtual const netenum_t* enumeration() const;
       virtual void dump(ostream&) const;
 
       virtual void expr_scan(struct expr_scan_t*) const;