add alert for macos

Signed-off-by: fufesou <shuanglongchen@yeah.net>

libs/hbb_common/examples/system_message.rs

@@ -0,0 +1,15 @@
+extern crate hbb_common;
+
+fn main() {
+    #[cfg(target_os = "linux")]
+    linux::system_message("test title", "test message", true).ok();
+    #[cfg(target_os = "macos")]
+    macos::alert(
+        "RustDesk".to_owned(),
+        "critical".to_owned(),
+        "test title".to_owned(),
+        "test message".to_owned(),
+        ["Ok".to_owned()].to_vec(),
+    )
+    .ok();
+}