Fix: C++14 requires out-of-line definition of member variable
diff --git a/c++/src/kj/async-io-unix.c++ b/c++/src/kj/async-io-unix.c++
index 706614e..aa845bc 100644
--- a/c++/src/kj/async-io-unix.c++
+++ b/c++/src/kj/async-io-unix.c++
@@ -804,6 +804,10 @@
   }
 };
 
+#if __linux__
+constexpr size_t AsyncStreamFd::MAX_SPLICE_LEN;
+#endif  // __linux__
+
 // =======================================================================================
 
 class SocketAddress {