fix: fix build on macos

src/platform_impl/macos.rs

@@ -0,0 +1,10 @@
+#![cfg(target_os = "macos")]
+
+use crate::SingleInstanceCallback;
+use tauri::{
+    plugin::{self, TauriPlugin},
+    Runtime,
+};
+pub fn init<R: Runtime>(f: Box<SingleInstanceCallback<R>>) -> TauriPlugin<R> {
+    plugin::Builder::new("single-instance").build()
+}