net: ip: Use proper flexible array

0 length array is a GNU extension. Use proper C99 flexible
array.

Signed-off-by: Flavio Ceolin <[email protected]>

Author: ceolin

subsys/net/ip/nbr.h

@@ -68,7 +68,7 @@ struct net_nbr {
 	/** Start of the data storage. Not to be accessed directly
 	 *  (the data pointer should be used instead).
 	 */
-	uint8_t __nbr[0] __net_nbr_align;
+	uint8_t __nbr[] __net_nbr_align;
 };
 
 /* This is an array of struct net_nbr + some additional data */