build(deps): bump deps/libtommath from 7f39a72 to 138309c

Bumps [deps/libtommath](https://github.com/libtom/libtommath) from `7f39a72` to `138309c`.
- [Release notes](https://github.com/libtom/libtommath/releases)
- [Commits](libtom/libtommath@7f39a72...138309c)

---
updated-dependencies:
- dependency-name: deps/libtommath
  dependency-type: direct:production
...

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