factor::numeric::gcd: Silence the (erroneous) dead code lint

src/uu/factor/src/numeric.rs

@@ -9,7 +9,10 @@
 
 use std::mem::swap;
 
-pub fn gcd(mut a: u64, mut b: u64) -> u64 {
+// This is incorrectly reported as dead code,
+//  presumably when included in build.rs.
+#[allow(dead_code)]
+pub(crate) fn gcd(mut a: u64, mut b: u64) -> u64 {
     while b > 0 {
         a %= b;
         swap(&mut a, &mut b);