fix(pf-clipboard): declare the libc dep the Linux backends call fully-qualified

wayland.rs (pipe2/poll on the paste pipes) and mutter.rs (fcntl un-nonblocking
on the transfer fd) reference libc:: inline — caught by the Linux leg.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2785,6 +2785,7 @@ dependencies = [
  "anyhow",
  "ashpd",
  "futures-util",
+ "libc",
  "punktfunk-core",
  "quinn",
  "tokio",

crates/pf-clipboard/Cargo.toml

@@ -27,6 +27,9 @@ tokio = { version = "1", features = ["rt", "rt-multi-thread", "sync", "time", "m
 # answer. Reusing ashpd's zbus re-export keeps one zbus version across the workspace.
 ashpd = "0.13"
 futures-util = "0.3"
+# Raw fd plumbing on the paste pipes: `pipe2(O_CLOEXEC)` + `poll` on the data-control receive
+# side, `fcntl` un-nonblocking on Mutter's transfer fd.
+libc = "0.2"
 wayland-client = "0.31"
 # `staging`: `ext_data_control_v1` (the session-clipboard protocol) ships in the staging set.
 wayland-protocols = { version = "0.32", features = ["client", "staging"] }