added stop function

src/ntp_client.cpp

@@ -37,3 +37,7 @@ void NtpClient::setDateTime() {
         sleep_ms(500);
     }
 }
+
+void NtpClient::stop() {
+    sntp_stop();
+}

src/ntp_client.h

@@ -9,6 +9,7 @@ extern "C" void set_system_time(u32_t sec);
 class NtpClient {
   public:
     static void setDateTime();
+    static void stop();
 
   private:
     NtpClient() {} // Disallow creating an instance of this object