lwip: Drop include in coap_io.h

The include is not required, as the `struct ud_pcb *` pointer doesn't
require the struct to be known yet, but caused trouble when the include
led to an inclusion loop back again to libcoap via the lwippools. (The
loop was caught by include guards and caused undeclared type errors
because the later parts of the headers were not yet processed).

include/coap/coap_io.h

@@ -15,10 +15,6 @@
 
 #include "address.h"
 
-#ifdef WITH_LWIP
-# include <lwip/udp.h>
-#endif
-
 /**
  * Abstract handle that is used to identify a local network interface.
  */

src/coap_io_lwip.c

@@ -10,6 +10,7 @@
 #include "coap_config.h"
 #include "mem.h"
 #include "coap_io.h"
+#include <lwip/udp.h>
 
 void coap_packet_populate_endpoint(coap_packet_t *packet, coap_endpoint_t *target)
 {