macros4.rs: Add rustfmt::skip to prevent auto-fix.

The `macros4.rs` challenge can automatically be solved by rustfmt without the user noticing.

Adding `#[rustfmt::skip]` above the `macro_rules!` line fixes this issue.

exercises/macros/macros4.rs

@@ -3,6 +3,7 @@
 
 // I AM NOT DONE
 
+#[rustfmt::skip]
 macro_rules! my_macro {
     () => {
         println!("Check out my macro!");