LibCore: Use WinSock2 closesocket() in PosixSocketHelper::close()

Our Core::System::is_socket() helper is not very robust, as it was
incorrectly marking m_fd as a non-socket fd. This meant we were trying
to call CLoseHandle() on a socket which fails. We can just call
closesocket() directly given we are in an explicit socket-only context.

Libraries/LibCore/SocketWindows.cpp

@@ -137,7 +137,7 @@ void PosixSocketHelper::close()
 
     // shutdown is required for another end to receive FD_CLOSE
     shutdown(m_fd, SD_BOTH);
-    MUST(System::close(m_fd));
+    closesocket(m_fd);
     m_fd = -1;
 }