🔥 Remove a redundant import of crate itself

Otherwise clippy complains about it:

```rust
 error: the item `syn` is imported redundantly
  --> zvariant_derive/src/lib.rs:14:11
   |
14 | use syn::{self, DeriveInput};
   |           ^^^^ the item `syn` is already defined here
   |
   = note: `-D unused-imports` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_imports)]`

error: the item `syn` is imported redundantly
 --> zvariant_derive/src/type.rs:4:5
  |
4 |     self, spanned::Spanned, Attribute, Data, DataEnum, DeriveInput, Error, Fields, Generics, Ident,
  |     ^^^^ the item `syn` is already defined here

error: the item `syn` is imported redundantly
 --> zvariant_derive/src/value.rs:4:5
  |
4 |     self, spanned::Spanned, Attribute, Data, DataEnum, DeriveInput, Error, Expr, Fields, Generics,
  |     ^^^^ the item `syn` is already defined here

   Compiling aho-corasick v1.1.2
   Compiling memoffset v0.9.0
    Checking utf8parse v0.2.1
error: could not compile `zvariant_derive` (lib) due to 3 previous errors
warning: build failed, waiting for other jobs to finish...
Error: Process completed with exit code 101.
```

zvariant_derive/src/lib.rs

@@ -11,7 +11,7 @@
 )))]
 
 use proc_macro::TokenStream;
-use syn::{self, DeriveInput};
+use syn::DeriveInput;
 
 mod dict;
 mod r#type;

zvariant_derive/src/type.rs

@@ -1,7 +1,7 @@
 use proc_macro2::TokenStream;
 use quote::{quote, ToTokens};
 use syn::{
-    self, spanned::Spanned, Attribute, Data, DataEnum, DeriveInput, Error, Fields, Generics, Ident,
+    spanned::Spanned, Attribute, Data, DataEnum, DeriveInput, Error, Fields, Generics, Ident,
 };
 
 use crate::utils::*;

zvariant_derive/src/value.rs

@@ -1,7 +1,7 @@
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, ToTokens};
 use syn::{
-    self, spanned::Spanned, Attribute, Data, DataEnum, DeriveInput, Error, Expr, Fields, Generics,
+    spanned::Spanned, Attribute, Data, DataEnum, DeriveInput, Error, Expr, Fields, Generics,
     Ident, Lifetime, LifetimeDef,
 };