[clang-tidy] Disable misc-definitions-in-headers for declarations in anonymous namespaces

Anonymous namespaces are another way of providing internal linkage,
and the check already ignores other cases of internal linkage in
headers, via "static" or "const".

Anonymous namespaces in headers are definitely a source of pitfalls,
but it's not the responsibility of this check to cover that. Instead,
google-build-namespaces will specifically warn about this issue.

Fixes https://github.com/llvm/llvm-project/issues/61471

Differential Revision: https://reviews.llvm.org/D147379