[Perl/perl5] cb6b3b: Avoid g++ warnings from last-element initialisers ...

[Paul Evans via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: cb6b3bdbf08a1fd3b5f9c62fbf0b43efd85380ce
      
https://github.com/Perl/perl5/commit/cb6b3bdbf08a1fd3b5f9c62fbf0b43efd85380ce
  Author: Paul "LeoNerd" Evans <leon...@leonerd.org.uk>
  Date:   2023-07-28 (Fri, 28 Jul 2023)

  Changed paths:
    M builtin.c
    M class.c

  Log Message:
  -----------
  Avoid g++ warnings from last-element initialisers in recently-added code

gcc was happy with this, but g++ complained

```
  builtin.c:511:1: warning: missing initializer for member 
‘BuiltinFuncDescriptor::xsub’ [-Wmissing-field-initializers]
    511 | };
        | ^
```