Allow Clippy lint: cyclomatic complexity

Honestly this function is pretty clear, breaking it up makes it seem worse.

src/naive/time.rs

@@ -518,6 +518,7 @@ impl NaiveTime {
     ///            (from_hms(20, 4, 5), -86400));
     /// # }
     /// ~~~~
+    #[cfg_attr(feature = "cargo-clippy", allow(cyclomatic_complexity))]
     pub fn overflowing_add_signed(&self, mut rhs: OldDuration) -> (NaiveTime, i64) {
         let mut secs = self.secs;
         let mut frac = self.frac;