Do not call a proc macro helper before it is defined

See rust#79202 <rust-lang/rust#79202>

Signed-off-by: Luca Barbato <[email protected]>

src/main.rs

@@ -24,14 +24,14 @@ struct Args {
     manifest_path: Option<PathBuf>,
 }
 
+#[derive(StructOpt, Debug)]
 #[structopt(
     name = "cargo-ebuild",
     bin_name = "cargo",
     author,
     about = "Generates an ebuild for a given Cargo project",
     global_settings(&[AppSettings::ColoredHelp])
 )]
-#[derive(StructOpt, Debug)]
 enum Opt {
     #[structopt(name = "ebuild")]
     /// Generates an ebuild for a given Cargo project