Bump syn from 0.15.31 to 0.15.32
Bumps [syn](https://github.com/dtolnay/syn) from 0.15.31 to 0.15.32.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](https://github.com/dtolnay/syn/compare/0.15.31...0.15.32)
Signed-off-by: dependabot[bot] <support@dependabot.com>