build(deps): bump pkg-config from 1.5.5 to 1.5.6

Bumps [pkg-config](https://github.com/ruby-gnome/pkg-config) from 1.5.5 to 1.5.6.
- [Changelog](https://github.com/ruby-gnome/pkg-config/blob/master/NEWS)
- [Commits](ruby-gnome/pkg-config@1.5.5...1.5.6)

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

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

Gemfile.lock

@@ -43,7 +43,7 @@ GEM
       hashery (~> 2.0)
       ruby-rc4
       ttfunk
-    pkg-config (1.5.5)
+    pkg-config (1.5.6)
     racc (1.7.3)
     rainbow (3.1.1)
     rake (13.1.0)