Remove Default on structs with pointer fields

These can't be derived in our current MSRV of 1.71, as the impl was only
introduced later in 1.88.

This partially reverts aa8ff85.

Author: madsmtm

crates/header-translator/src/rust_type.rs

@@ -2993,8 +2993,16 @@ impl Ty {
                 // less of a breaking change if we change the fields to be
                 // `NonNull` in the future.
                 nullability: Nullability::Nullable,
+                pointee,
                 ..
-            } => true,
+                // TODO: Return `true` here always once MSRV is 1.88, that's
+                // when pointer types started implementing `Default` (which
+                // allows us to derive the `Default`).
+                //
+                // Alternatively, we could implement `Default` manually on
+                // these, but that's a bit of a hassle, so we won't bother for
+                // now.
+            } => matches!(&**pointee, Ty::Pointee(PointeeTy::Fn { .. })),
             // Only arrays up to size 32 implement Default.
             Self::Array {
                 element_type,