8274793: Suppress warnings on non-serializable non-transient instance fields in sun.net

Reviewed-by: alanb

src/java.base/share/classes/sun/net/www/http/KeepAliveCache.java

@@ -79,6 +79,7 @@ public class KeepAliveCache
 
     // This class is never serialized (see writeObject/readObject).
     private final ReentrantLock cacheLock = new ReentrantLock();
+    @SuppressWarnings("serial") // Type of field is not Serializable
     private Thread keepAliveTimer = null;
 
     /**