Call coap_startup() to remove warning

If coap_startup() is not called prior to any other coap_* function calls, a
warning is emitted.

```
coap_startup() should be called before any other coap_*() functions are
called
```

This proactively calls coap_startup() to ensure the library is initialized,
even though coap_new_context() will do so implicitly.

Signed-off-by: Daniel Mangum <georgedanielmangum@gmail.com>

src/golioth_coap_client.c

@@ -966,6 +966,9 @@ golioth_client_t golioth_client_create(const golioth_client_config_t* config) {
         time_t t;
         srand(time(&t));
 
+        // Initialize libcoap prior to any coap_* function calls.
+        coap_startup();
+
         _initialized = true;
     }