Update FlipperPlugin C++ code so it can be compiled for Windows

Summary:
I am working in Horizon Worlds and I would like to integrate Flipper with HzW app.
Currently FlipperPlugin C++ code won't compile on Windows since it uses Linux-only headers like `netdb.h` and `sys/fcntl.h`, I posted here and looks like it is not currently supported: https://fb.workplace.com/groups/flippersupport/posts/1704837183330266

The problem seem to be in only in `FlipperConnectionEndpointVerifier.cpp`, and I'm updating it to make it compatible with Windows.

Also apparently there's some issue with `#include` of few files and namespaces, leading to "struct redefinition" errors where `#pragma once` does not help https://fb.workplace.com/groups/474291069286180/posts/25313067014981908/

Solving it with manual #define

Reviewed By: lblasa

Differential Revision: D50337573

fbshipit-source-id: affdf1aee2b9dfe615227827fedf324a5f17d8b0

xplat/Flipper/FlipperScheduler.h

@@ -6,6 +6,8 @@
  */
 
 #pragma once
+#ifndef _FLIPPERSCHEDULER_H_
+#define _FLIPPERSCHEDULER_H_
 
 #include <functional>
 
@@ -24,3 +26,4 @@ struct Scheduler {
 
 } // namespace flipper
 } // namespace facebook
+#endif