[vm] Fix -O0 build after CL 101222

const objects that are declared in .h files are supposed to have
out-of-line definitions in their corresponding .cc file. It seems like
CL 101222 happened to expose this for Message::kIllegalPort.

Change-Id: I9656037581eb6406eaf3289dfcf705db90862a26
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/102122
Commit-Queue: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Régis Crelier <regis@google.com>

runtime/vm/message.cc

@@ -11,6 +11,8 @@
 
 namespace dart {
 
+const Dart_Port Message::kIllegalPort;
+
 Message::Message(Dart_Port dest_port,
                  uint8_t* snapshot,
                  intptr_t snapshot_length,