Changed process id method to use getmypid for compatibility with Windows

Author: bstoney

src/JupyterBroker.php

@@ -77,7 +77,7 @@ public function send(
         );
 
         if (null !== $this->logger) {
-            $this->logger->debug('Sent message', ['processId' => posix_getpid(), 'message' => $finalMsg]);
+            $this->logger->debug('Sent message', ['processId' => getmypid(), 'message' => $finalMsg]);
         }
 
         $stream->send($finalMsg);