Explicitly default pNext fields again

This undoes #5, as there are still some structs which do not properly
set this value.

generator/vulkan/parse.zig

@@ -220,6 +220,11 @@ fn parseContainer(allocator: Allocator, ty: *xml.Element, is_union: bool) !regis
     for (members) |*member| {
         const member_elem = it.next().?;
         try parsePointerMeta(.{ .container = members }, &member.field_type, member_elem);
+
+        // pNext isn't always properly marked as optional, so just manually override it,
+        if (mem.eql(u8, member.name, "pNext")) {
+            member.field_type.pointer.is_optional = true;
+        }
     }
 
     return registry.Declaration{