Bump firrtl from `899f1f6` to `b6fd24c` (#157)

Bumps [firrtl](https://github.com/freechipsproject/firrtl) from `899f1f6` to `b6fd24c`.
- [Release notes](https://github.com/freechipsproject/firrtl/releases)
- [Commits](https://github.com/freechipsproject/firrtl/compare/899f1f6efcbd84c0bb9ef0529e024dd36de2bf4f...b6fd24cfea04e6db019fde61dba349dbfe45bf9c

)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>