coap-rd.c: include uthash.h

The coap-rd example uses uthash.h from the libcoap core. As uthash.h
is not part of the public libcoap API, it must be included explicitly.

examples/coap-rd.c

@@ -42,6 +42,7 @@
 #endif
 
 #include <coap3/coap.h>
+#include "uthash.h"
 
 #define COAP_RESOURCE_CHECK_TIME 2