Bump pbkdf2 in /smart_contracts/ethereum/solidity_firefly

Bumps [pbkdf2](https://github.com/browserify/pbkdf2) from 3.1.2 to 3.1.5.
- [Changelog](https://github.com/browserify/pbkdf2/blob/master/CHANGELOG.md)
- [Commits](browserify/pbkdf2@v3.1.2...v3.1.5)

---
updated-dependencies:
- dependency-name: pbkdf2
  dependency-version: 3.1.5
  dependency-type: indirect
...

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

Author: dependabot[bot]