Allows Union Types in Parameter Fields

If a union type was referenced as a parameter this would
fail.

Signed-off-by: Ethan Mahintorabi <ethanmoon@google.com>

systemverilog-plugin/UhdmAst.cc

diff --git a/systemverilog-plugin/UhdmAst.cc b/systemverilog-plugin/UhdmAst.cc
index 0be44d7..bfc20b9 100644
--- a/systemverilog-plugin/UhdmAst.cc
+++ b/systemverilog-plugin/UhdmAst.cc
@@ -4835,6 +4835,7 @@
             shared.report.mark_handled(typespec_h);
             break;
         }
+        case vpiUnionTypespec:
         case vpiStructTypespec: {
             visit_one_to_one({vpiTypespec}, obj_h, [&](AST::AstNode *node) {
                 if (node && !node->str.empty()) {