Fix noop deref warning

[LegNeato]

Without this change compiling shows a warning:

warning: call to `.deref()` on a reference in this situation does nothing
    --> c2rust-transpile/src/cfg/mod.rs:2252:72
     |
2252 | ...to_string(pat.deref());
     |                 ^^^^^^^^ help: remove this redundant call
     |
     = note: the type `syn::Pat` does not implement `Deref`, so calling `deref` on `&syn::Pat` copies the reference, which does not do anything and can be removed
     = note: `#[warn(noop_method_call)]` on by default