Build failures after updates

I get here some build failures after commit:

https://github.com/google/quiche/commit/04700ac7fe5b3865f7dcbd0d230fa3e4fe4036ec

I get some compiler failures with GNU 13.3.0
```
In file included from /home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.cc:5:
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:77:36: error: ‘constexpr’ static data member ‘string’ must have an initializer
77 | inline static constexpr string_t string;
| ^~~~~~
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:77:36: error: no matching function for call to ‘quiche::structured_headers::Item::string_t::string_t()’
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:74:10: note: candidate: ‘constexpr quiche::structured_headers::Item::string_t::string_t(const quiche::structured_headers::Item::string_t&)’
74 | struct string_t {
| ^~~~~~~~
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:74:10: note: candidate expects 1 argument, 0 provided
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:74:10: note: candidate: ‘constexpr quiche::structured_headers::Item::string_t::string_t(quiche::structured_headers::Item::string_t&&)’
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:74:10: note: candidate expects 1 argument, 0 provided
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:82:35: error: ‘constexpr’ static data member ‘token’ must have an initializer
82 | inline static constexpr token_t token;
| ^~~~~
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:82:35: error: no matching function for call to ‘quiche::structured_headers::Item::token_t::token_t()’
/home/runner/work/webtransport/webtransport/transports/http3-quiche/third_party/quiche/quiche/common/structured_headers.h:79:10: note: candidate: ‘constexpr quiche::structured_headers::Item::token_t::token_t(const quiche::structured_headers::Item::token_t&)’
79 | struct token_t {
```

Start by comparing commit 04700ac7fe5b3865f7dcbd0d230fa3e4fe4036ec with the current code, then inspect third_party/quiche/quiche/common/structured_headers.h around lines 74-82. Reproduce the failure with GNU 13.3.0 and verify that the affected build completes without the reported constexpr errors.

Written by the indexing model from the issue text.