Fix old GCC build.

c++/src/kj/one-of.h

@@ -91,15 +91,11 @@ public:
   }
 
   template <uint i>
-  KJ_NORETURN(void allHandled()) {
+  KJ_NORETURN(void allHandled());
   // After a series of if/else blocks handling each variant of the OneOf, have the final else
   // block call allHandled<n>() where n is the number of variants. This will fail to compile
   // if new variants are added in the future.
 
-    static_assert(i == sizeof...(Variants), "new OneOf variants need to be handled here");
-    KJ_UNREACHABLE;
-  }
-
 private:
   uint tag;
 
@@ -169,6 +165,17 @@ private:
   }
 };
 
+template <typename... Variants>
+template <uint i>
+void OneOf<Variants...>::allHandled() {
+  // After a series of if/else blocks handling each variant of the OneOf, have the final else
+  // block call allHandled<n>() where n is the number of variants. This will fail to compile
+  // if new variants are added in the future.
+
+  static_assert(i == sizeof...(Variants), "new OneOf variants need to be handled here");
+  KJ_UNREACHABLE;
+}
+
 }  // namespace kj
 
 #endif  // KJ_ONE_OF_H_