Fix clippy dead code warning

src/backend/linux_raw/reg.rs

@@ -45,7 +45,9 @@ pub(super) trait FromAsm: private::Sealed {
 /// pointer types. They need a type to point to, so we define a custom private
 /// type, to prevent it from being used for anything else.
 #[repr(transparent)]
-pub(super) struct Opaque(c::c_void);
+pub(super) struct Opaque {
+    _unused: c::c_void,
+}
 
 // Argument numbers.
 pub(super) struct A0(());