Fix compilation warnings in v0.7.1

[jovfer]

Compiling sd-jwt-rs v0.7.1 (/Users/sminaev/projects/sd-jwt-rs/target/package/sd-jwt-rs-0.7.1)
warning: struct SDJWTHasSDClaimException is never constructed
--> src/lib.rs:36:19
|
36 | pub(crate) struct SDJWTHasSDClaimException(String);
| ^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: #[warn(dead_code)] on by default

warning: sd-jwt-rs (lib) generated 1 warning