zoap: Include net/net_ip.h when sockaddr is used

Add include file net/net_ip.h as zoap header files use struct
sockaddr.

Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>
diff --git a/include/net/zoap.h b/include/net/zoap.h
index cabc967..ff99a1d 100644
--- a/include/net/zoap.h
+++ b/include/net/zoap.h
@@ -16,6 +16,7 @@
 #include <zephyr/types.h>
 #include <stddef.h>
 #include <stdbool.h>
+#include <net/net_ip.h>
 
 #include <misc/slist.h>