chore: Fix some violations of 'needless_pass_by_ref_mut' lint (#18795)

While this lint is allow-by-default, it seems pretty useful to get rid
of mutable borrows when they're not needed.

Closes #ISSUE

Release Notes:

- N/A

crates/gpui/src/platform/mac/platform.rs

@@ -1432,7 +1432,7 @@ impl UTType {
         self.0
     }
 
-    fn inner_mut(&mut self) -> *mut Object {
+    fn inner_mut(&self) -> *mut Object {
         self.0 as *mut _
     }
 }