Add #[inline] to impl Zero for ()

Follow-up to #8155

Author: stepancheg

src/libstd/nil.rs

@@ -55,7 +55,9 @@ impl TotalEq for () {
 
 #[cfg(not(test))]
 impl Zero for () {
+    #[inline]
     fn zero() -> () { () }
+    #[inline]
     fn is_zero(&self) -> bool { true }
 }