gui: don't warn about wsl helper function on unix

Technically is only used on windows, but I'm fine with having
it built and tested on all platforms.

wezterm-gui/src/overlay/launcher.rs

@@ -60,6 +60,7 @@ struct WslDistro {
 /// by looking at the tabulated output headers, determining
 /// where the columns are and then collecting the information
 /// into a hashmap and then grokking from there.
+#[allow(dead_code)]
 fn parse_wsl_distro_list(output: &str) -> Vec<WslDistro> {
     let lines = output.lines().collect::<Vec<_>>();