third_party:quic_ko_wrapper Fix compilation with clang-20

third_party:quic_ko_wrapper Fix compilation with clang-20

Rework the code to remove the non portable variable length array in the union.
Based on the code in wrap_msghdr_add_cmsghdr().

NOTE: this removes the 64 bit alignment, which I think should be ok.

./../third_party/quic_ko_wrapper/quic_ko_wrapper.c:3523:11: error:
   fields must have a constant size: 'variable length array in structure'
   extension will never be supported
  3523 |                 uint8_t cmbuf[cmspace];
       |                         ^
1 error generated.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Björn Baumbach <bb@sernet.de>
Autobuild-Date(master): Wed Aug 20 15:04:34 UTC 2025 on atb-devel-224