Fix singlecore warning

src/singlecore.rs

@@ -16,7 +16,7 @@ impl Worker {
     // We don't expose this outside the library so that
     // all `Worker` instances have the same number of
     // CPUs configured.
-    pub(crate) fn new_with_cpus(cpus: usize) -> Worker {
+    pub(crate) fn new_with_cpus(_cpus: usize) -> Worker {
         Worker {
             cpus: 1,
         }