build: bump pkg-config from 0.3.19 to 0.3.20

Bumps [pkg-config](https://github.com/rust-lang/pkg-config-rs) from 0.3.19 to 0.3.20.
- [Release notes](https://github.com/rust-lang/pkg-config-rs/releases)
- [Changelog](https://github.com/rust-lang/pkg-config-rs/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-lang/pkg-config-rs/compare/0.3.19...0.3.20)

---
updated-dependencies:
- dependency-name: pkg-config
  dependency-type: indirect
  update-type: version-update:semver-patch
...

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

Cargo.lock

@@ -691,9 +691,9 @@ dependencies = [
 
 [[package]]
 name = "pkg-config"
-version = "0.3.19"
+version = "0.3.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3831453b3449ceb48b6d9c7ad7c96d5ea673e9b470a1dc578c2ce6521230884c"
+checksum = "7c9b1041b4387893b91ee6746cddfc28516aff326a3519fb2adf820932c5e6cb"
 
 [[package]]
 name = "pnet"