util: Suppress a warning by adding CONST_CAST

Suppress a gcc warning which was introduced by
commit e0b48482c16b6eaa7f14d8c7e7c6275528881b9e.
("util: create a copy of program_name")
I guess MSVC doesn't have a corresponding warning.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Lorand Jakab <lojakab@cisco.com>