gnutls: fix disable zstd

Configure.ac syntax changed to:
  Old: --without-libbrotli --without-libzstd (also --with-*)
  New: --without-brotli --without-zstd (also --with-*)
  https://github.com/gnutls/gnutls/commit/6b794e49d1a14e43f9e08023f958364712c3c89a

Fixes: 6385813d

 ("gnutls: update to 3.7.5")

Signed-off-by: Nick Hainke <vincent@systemli.org>