tls-gnutls: Make loop variable unsigned integer

Fix the error below (gcc (Ubuntu 5.4.0-6ubuntu1~16.04.11) 5.4.0
20160609):

    Compiling tls.c...
    In file included from tls.c:39:0:
    tls-gnutls.c: In function ‘httpCredentialsAreValidForName’:
    tls-gnutls.c:420:56: error: conversion to ‘int’ from ‘unsigned int’ may change the sign of the result [-Werror=sign-conversion]
               if (!gnutls_x509_crl_get_crt_serial(tls_crl, (unsigned)i, rserial, &r
                                                            ^

Fixes: apple#5532

cups/tls-gnutls.c

@@ -398,7 +398,7 @@ httpCredentialsAreValidForName(
 
     if (result)
     {
-      int		i,		/* Looping var */
+      unsigned int	i,		/* Looping var */
 			count;		/* Number of revoked certificates */
       unsigned char	cserial[1024],	/* Certificate serial number */
 			rserial[1024];	/* Revoked serial number */
@@ -417,7 +417,7 @@ httpCredentialsAreValidForName(
         for (i = 0; i < count; i ++)
 	{
 	  rserial_size = sizeof(rserial);
-          if (!gnutls_x509_crl_get_crt_serial(tls_crl, (unsigned)i, rserial, &rserial_size, NULL) && cserial_size == rserial_size && !memcmp(cserial, rserial, rserial_size))
+          if (!gnutls_x509_crl_get_crt_serial(tls_crl, i, rserial, &rserial_size, NULL) && cserial_size == rserial_size && !memcmp(cserial, rserial, rserial_size))
 	  {
 	    result = 0;
 	    break;