fix: correct lean_kernel_diag_is_enabled declaration to match generated C

[FawadHa1der]

This PR fixes the lean_kernel_diag_is_enabled C++ declaration to match generated C. This issue is manifesting only for wasm64 output but it is an issue nonetheless. C++ compiled to wasm64 via emscripten incorrectly translates based on the C++ external declaration.

Closes issue 14865

[FawadHa1der]

maintainers apply the appropriate labels please. may be changelog-no (no observable change on supported targets) since wasm64 is not supported at the moment.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase f6c7d68c7fc27e3b60585f30740c7d141d3c5b36 --onto 16e77c407779fde9a649adf3478204d1915371a3. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-21 16:49:58)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase f6c7d68c7fc27e3b60585f30740c7d141d3c5b36 --onto 16e77c407779fde9a649adf3478204d1915371a3. You can force reference manual CI using the force-manual-ci label. (2026-08-21 16:50:00)