Clipboard: TEXT_ONLY is built on Linux only

The constant is used only by the Linux Wayland arms, so on macOS it is
dead code and `cargo clippy -p vectorcraft --all-targets -- -D warnings`
fails with "constant `TEXT_ONLY` is never used". Gate it with
`#[cfg(target_os = "linux")]`, as the `Wayland` variant beside it is.

apps/vectorcraft/src/clipboard.rs

@@ -339,6 +339,7 @@ mod portable {
     }
 
     /// What a write of a picture says where the clipboard carries text only.
+    #[cfg(target_os = "linux")]
     const TEXT_ONLY: &str = "this clipboard carries text only";
 
     impl Backend {