'http_request_t' has a previous declaration as 'typedef struct http_request_t http_request_t'

Veryfing code using this library in Particle Build fails with this error:

```
HttpClient/HttpClient.h:44:3: error: 'http_request_t' has a previous declaration as 'typedef struct http_request_t http_request_t'
 } http_request_t
```
