Bump libwebp-sys from 0.9.2 to 0.9.4 in /server (#820)

Bumps [libwebp-sys](https://github.com/NoXF/libwebp-sys) from 0.9.2 to 0.9.4.
- [Commits](https://github.com/NoXF/libwebp-sys/commits)

---
updated-dependencies:
- dependency-name: libwebp-sys
  dependency-type: indirect
...

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