libgcrypt: Avoid -Wsign-compare in rijndael do_setkey()

Avoid a warning

  lib/libgcrypt-grub/cipher/rijndael.c:352:21: warning:
  comparison of integer expressions of different signedness:
  ‘int’ and ‘unsigned int’ [-Wsign-compare]
    352 |       for (i = 0; i < keylen; i++)
        |

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/lib/libgcrypt/cipher/rijndael.c

@@ -181,7 +181,8 @@ do_setkey (RIJNDAEL_context *ctx, const byte *key, const unsigned keylen)
   static int initialized = 0;
   static const char *selftest_failed=0;
   int rounds;
-  int i,j, r, t, rconpointer = 0;
+  unsigned int i;
+  int j, r, t, rconpointer = 0;
   int KC;
   union
   {