[libcxx-commits] [libcxx] Remove incorrect forward-declaration of lgamma_r header (PR #80979)

[Louis Dionne via libcxx-commits]

ldionne wrote:

This fails on the Apple CI with:

```
/Users/libcxx-buildkite-agent/libcxx.buildkite-agent/builds/f1-1-macminivault-com/llvm-project/libcxx-ci/build/apple-system/arm64/include/c++/v1/__random/binomial_distribution.h:105:10: error: use of undeclared identifier 'lgamma_r'
  return lgamma_r(__d, &__sign);
         ^
```

https://github.com/llvm/llvm-project/pull/80979