Squelch a Coverity warning.

See Coverity CID 1324572 for tcpdump - the unsigned octets from the IPv6
address were getting extended to ints as a result of getting shifted
left by an int, so make the loop counter unsigned.

addrtostr.c

@@ -113,7 +113,7 @@ addrtostr6 (const void *src, char *dst, size_t size)
     long len;
   } best, cur;
   u_long words [IN6ADDRSZ / INT16SZ];
-  int    i;
+  u_int  i;
 
   /* Preprocess:
    *  Copy the input (bytewise) array into a wordwise array.