Fix a compile warning on OS X 10.6

Also update the changes file to contain a note on which bug was
fixed by this.

changes/bug1384

@@ -1,5 +1,5 @@
   o Minor features:
     - Warn when the same option is provided more then once in a torrc file,
       on the command line, or in a single SETCONF statement, and option
-      is one that only accepts a single value.
+      is one that only accepts a single value. Closes bug 1384.
 

src/or/config.c

@@ -1761,7 +1761,7 @@ config_assign_line(config_format_t *fmt, or_options_t *options,
                        var->type != CONFIG_TYPE_LINELIST_S)) {
     /* We're tracking which options we've seen, and this option is not
      * supposed to occur more than once. */
-    int var_index = var - fmt->vars;
+    int var_index = (int)(var - fmt->vars);
     if (bitarray_is_set(options_seen, var_index)) {
       log_warn(LD_CONFIG, "Option '%s' used more than once; all but the last "
                "value will be ignored.", var->name);