Fix some clippy warning from Rust 1.78.0 (#4444)

examples/file_dialog/src/main.rs

@@ -78,7 +78,7 @@ impl eframe::App for MyApp {
         // Collect dropped files:
         ctx.input(|i| {
             if !i.raw.dropped_files.is_empty() {
-                self.dropped_files = i.raw.dropped_files.clone();
+                self.dropped_files.clone_from(&i.raw.dropped_files);
             }
         });
     }