undef poll at the end if CPPHTTPLIB_USE_POLL (yhirose#1427)

* undef poll at the end if CPPHTTPLIB_USE_POLL

* win32 only

Author: artpaul

httplib.h

@@ -8405,4 +8405,8 @@ inline SSL_CTX *Client::ssl_context() const {
 
 } // namespace httplib
 
+#if defined(_WIN32) && defined(CPPHTTPLIB_USE_POLL)
+#undef poll
+#endif
+
 #endif // CPPHTTPLIB_HTTPLIB_H