Bump signal-hook from 0.3.14 to 0.3.15

Bumps [signal-hook](https://github.com/vorner/signal-hook) from 0.3.14 to 0.3.15.
- [Release notes](https://github.com/vorner/signal-hook/releases)
- [Changelog](https://github.com/vorner/signal-hook/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vorner/signal-hook/compare/v0.3.14...v0.3.15)

---
updated-dependencies:
- dependency-name: signal-hook
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

bridges/relays/bin-substrate/Cargo.toml

@@ -17,7 +17,7 @@ num-format = "0.4"
 num-traits = "0.2"
 rbtag = "0.3"
 structopt = "0.3"
-signal-hook = "0.3.14"
+signal-hook = "0.3.15"
 signal-hook-async-std = "0.2.2"
 strum = { version = "0.24.1", features = ["derive"] }