rust: macros: vtable: fix HAS_* redefinition (gen_const_name)

If we define the same function name twice in a trait (using `#[cfg]`),
the `vtable` macro will redefine its `gen_const_name`, e.g. this will
define `HAS_BAR` twice:

    #[vtable]
    pub trait Foo {
        #[cfg(CONFIG_X)]
        fn bar();

        #[cfg(not(CONFIG_X))]
        fn bar(x: usize);
    }

Fixes: b44becc ("rust: macros: add `#[vtable]` proc macro")
Signed-off-by: Qingsong Chen <[email protected]>
Reviewed-by: Andreas Hindborg <[email protected]>
Reviewed-by: Gary Guo <[email protected]>
Reviewed-by: Sergio González Collado <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

rust/macros/vtable.rs

@@ -74,6 +74,7 @@ pub(crate) fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
                 const {gen_const_name}: bool = false;",
             )
             .unwrap();
+            consts.insert(gen_const_name);
         }
     } else {
         const_items = "const USE_VTABLE_ATTR: () = ();".to_owned();