Add missing proof_for_contract to mut_ptr::offset_from harness (#358)

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under the terms of the Apache 2.0 and MIT licenses.

Co-authored-by: Michael Tautschnig <[email protected]>

Author: carolynzech

library/core/src/ptr/mut_ptr.rs

@@ -2613,6 +2613,7 @@ mod verify {
             }
 
             // Below function is for large arrays
+            #[kani::proof_for_contract(<*mut $type>::offset_from)]
             pub fn $proof_name2() {
                 const gen_size: usize = mem::size_of::<$type>();
                 let mut generator1 = PointerGenerator::<{ gen_size * ARRAY_LEN }>::new();