Bump libm from 0.2.13 to 0.2.15

Bumps [libm](https://github.com/rust-lang/compiler-builtins) from 0.2.13 to 0.2.15.
- [Release notes](https://github.com/rust-lang/compiler-builtins/releases)
- [Changelog](https://github.com/rust-lang/compiler-builtins/blob/master/.release-plz.toml)
- [Commits](rust-lang/compiler-builtins@libm-v0.2.13...libm-v0.2.15)

---
updated-dependencies:
- dependency-name: libm
  dependency-version: 0.2.15
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]