Added feature flag and pkgconfig-entry for the Xfixes-library to the x11-crate

[m0rphism]

trafficstars

See #158 for details.

• [x] Tested on an x11 machine
• [x] Compilation warnings were addressed
• [x] cargo fmt has been run on this branch
• [x] cargo doc builds successfully
• [ ] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [ ] Updated documentation to reflect any user-facing changes
• [ ] Created or updated an example program if it would help users understand this functionality

[AltF02]

Could you also add an entry in CHANGELOG.md? Cheers