Add missing include guards to NTPClient.h

The header file [`NTPClient.h`](https://github.com/ARMmbed/ntp-client/blob/e919cfb14e2029efafc0d1288a0223cbd09811f0/NTPClient.h) does not have an include guard (`#ifndef`/`#define` or `#pragma once`).
Without include guards, including this header multiple times in different files causes duplicate definition errors during compilation.
I suggest adding a `#pragma once` at the beginning of `NTPClient.h`.

Start with NTPClient.h at the repository root and confirm how the header is currently structured. Add the requested include guard, then verify that including NTPClient.h multiple times no longer causes duplicate definition errors during compilation.

Written by the indexing model from the issue text.