Fix signedness warning in glib/tests/base64.c

glib/tests/base64.c:28:20: error: comparison of integer expressions of
different signedness: ‘gsize’ {aka ‘long unsigned int’} and ‘gint’ {aka ‘int’}

   28 |   while (input_len < length)
      |                    ^

glib/tests/base64.c

@@ -9,7 +9,7 @@ static guchar data[DATA_SIZE];
 
 static void
 test_incremental (gboolean line_break,
-                  gint     length)
+                  gsize    length)
 {
   char *p;
   gsize len, decoded_len, max, input_len, block_size;