Add libc dependency for unix targets

This commit adds the `libc` dependency to the `Cargo.toml` file for
Unix-based targets. This dependency is required by the `ffmpeg` command
execution within the `recorder` module, specifically for handling
process management and signaling on Unix-like systems.

client-desktop/Cargo.toml

@@ -19,6 +19,9 @@ directories = "5"
 thiserror = "1"
 which = "6"
 
+[target.'cfg(unix)'.dependencies]
+libc = "0.2"
+
 [dev-dependencies]
 tempfile = "3"
 tokio = { workspace = true, features = ["test-util"] }