libwacom: 0.26 -> 0.28

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/yi7myg1fay4my7dl64nbj0mlv2qdma6z-libwacom-0.28/bin/libwacom-list-local-devices -h` got 0 exit code
- ran `/nix/store/yi7myg1fay4my7dl64nbj0mlv2qdma6z-libwacom-0.28/bin/libwacom-list-local-devices --help` got 0 exit code
- ran `/nix/store/yi7myg1fay4my7dl64nbj0mlv2qdma6z-libwacom-0.28/bin/libwacom-list-local-devices help` got 0 exit code
- found 0.28 with grep in /nix/store/yi7myg1fay4my7dl64nbj0mlv2qdma6z-libwacom-0.28
- found 0.28 in filename of file in /nix/store/yi7myg1fay4my7dl64nbj0mlv2qdma6z-libwacom-0.28