conflicting return type specified for virtual int Client::read(uint8_t *buf, size_t size)

Client inherits from Stream but read is declared in Client.h as:

```C++
virtual int read(uint8_t *buf, size_t size) = 0;
```

whilst the parent declaration in Stream.h is:

```C++
virtual size_t read(uint8_t *buffer, size_t size);
```

Using the Adafruit_MQTT library generates the above error. Changing from int to size_t as the return type in Client.h fixes the problem.
