shader_recompiler: GCC fixes

Fixes members of unnamed union not being accessible, and one function
without a declaration.

src/shader_recompiler/frontend/ir/post_order.cpp

@@ -20,7 +20,7 @@ BlockList PostOrder(const AbstractSyntaxNode& root) {
     if (root.type != AbstractSyntaxNode::Type::Block) {
         throw LogicError("First node in abstract syntax list root is not a block");
     }
-    Block* const first_block{root.block};
+    Block* const first_block{root.data.block};
     visited.insert(first_block);
     block_stack.push_back(first_block);