address.h: added missing includes

libcoap.h must be included to ensure that platform-specific headers
for the target system are available.

include/coap/address.h

@@ -18,6 +18,8 @@
 #include <assert.h>
 #include <stdint.h>
 #include <string.h>
+#include <sys/types.h>
+#include "libcoap.h"
 
 #ifdef WITH_LWIP
 #include <lwip/ip_addr.h>