src: add default: case to silence compiler warning

This fails compilation on at least one platform because there is no
`default:` case, despite all currently possible enum values being
listed.

Fix that by adding a default message that won’t be used unless V8
introduces new enum values.

Refs: c7eeef5#commitcomment-39228519

PR-URL: #33451
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

src/node.cc

@@ -243,7 +243,7 @@ static void AtomicsWaitCallback(Isolate::AtomicsWaitEvent event,
                                 void* data) {
   Environment* env = static_cast<Environment*>(data);
 
-  const char* message;
+  const char* message = "(unknown event)";
   switch (event) {
 #define V(key, msg)                         \
     case Isolate::AtomicsWaitEvent::key:    \