rpc_bindings.rs: Fix mismatched_lifetime_syntaxes error

[jinetty]

After Rust 1.89.0 version, Lifetime elision lint added. This creates error: hiding a lifetime that's elided elsewhere is confusing error and requires <'_> for obvious time validation.

error: hiding a lifetime that's elided elsewhere is confusing
  --> sudo\src\rpc_bindings.rs:16:19
   |
16 |     pub fn new(s: &str) -> Utf8Str {
   |                   ^^^^     ------- the same lifetime is hidden here
   |                   |
   |                   the lifetime is elided here
   |
   = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
   = note: `-D mismatched-lifetime-syntaxes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(mismatched_lifetime_syntaxes)]`
help: use `'_` for type paths
   |
16 |     pub fn new(s: &str) -> Utf8Str<'_> {
   |                                   ++++

error: could not compile `sudo` (bin "sudo") due to 1 previous error