Fix windows warning introduced by 0808ed83f9

This will fix the warning
   "/src/or/config.c:6854:48: error: unused parameter 'group_readable'"
that I introduced while fixing 12864.

Bug not in any released version of Tor.

src/or/config.c

@@ -6871,6 +6871,8 @@ init_cookie_authentication(const char *fname, const char *header,
       log_warn(LD_FS,"Unable to make %s group-readable.", escaped(fname));
     }
   }
+#else
+  (void) group_readable;
 #endif
 
   /* Success! */