FIX: Fix future compat warning with raw pointer casts

```
warning: the type of this value must be known in this context
   --> src/graphmap.rs:765:11
    |
765 |         a.cmp(&b)
    |           ^^^
    |
    = note: #[warn(tyvar_behind_raw_pointer)] on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>
```
1 file changed