Fix a typo

1. According to the UTF-8 spec, ASCII character's charcode is from
   0 to 127 inclusively.

2. Any charcode which is greater than or equal to 128 will be
   a multi-byte character.

gtk/gtkcssparser.c

@@ -373,7 +373,7 @@ _gtk_css_parser_read_char (GtkCssParser *parser,
       parser->data++;
       return TRUE;
     }
-  if ((*(guchar *) parser->data) >= 127)
+  if ((*(guchar *) parser->data) >= 128)
     {
       gsize len = g_utf8_skip[(guint) *(guchar *) parser->data];