src: add NODE_EXTERN to class definition

This should be part of public class definitions for Windows
embedders who use a DLL interface for accessing Node.

PR-URL: #22559
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: addaleax

src/node.h

@@ -232,7 +232,7 @@ NODE_EXTERN void FreeArrayBufferAllocator(ArrayBufferAllocator* allocator);
 class IsolateData;
 class Environment;
 
-class MultiIsolatePlatform : public v8::Platform {
+class NODE_EXTERN MultiIsolatePlatform : public v8::Platform {
  public:
   virtual ~MultiIsolatePlatform() { }
   // Returns true if work was dispatched or executed. New tasks that are