Fix link errors.

c++/src/capnp/capability.c++

@@ -105,8 +105,6 @@ kj::Promise<void> ClientHook::whenResolved() {
   }
 }
 
-const uint ClientHook::NULL_CAPABILITY_BRAND = 0;
-
 // =======================================================================================
 
 static inline uint firstSegmentSize(kj::Maybe<MessageSize> sizeHint) {

c++/src/capnp/layout.c++

@@ -43,6 +43,14 @@ void setGlobalBrokenCapFactoryForLayoutCpp(BrokenCapFactory& factory) {
   // is ready for it.  May be called multiple times but always with the same value.
   __atomic_store_n(&brokenCapFactory, &factory, __ATOMIC_RELAXED);
 }
+
+}  // namespace _ (private)
+
+const uint ClientHook::NULL_CAPABILITY_BRAND = 0;
+// Defined here rather than capability.c++ so that we can safely call isNull() in this file.
+
+namespace _ {  // private
+
 #endif  // !CAPNP_LITE
 
 // =======================================================================================