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: b44becc5ee80 ("rust: macros: add `#[vtable]` proc macro")
Signed-off-by: Qingsong Chen <[e-mail]>
Reviewed-by: Andreas Hindborg <[e-mail]>
Reviewed-by: Gary Guo <[e-mail]>
Reviewed-by: Sergio González Collado <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Miguel Ojeda <[e-mail]>

rust/macros/vtable.rs

···
                 const {gen_const_name}: bool = false;",
             )
             .unwrap();
+            consts.insert(gen_const_name);
         }
     } else {
         const_items = "const USE_VTABLE_ATTR: () = ();".to_owned();