stringutils: include stdarg for va_list
stringutils: include stdarg for va_list

Fixes:
 - http://autobuild.buildroot.org/results/0b90e7dca2984652842832a41abad93ac49a9b86

Signed-off-by: Fabrice Fontaine <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>

hallyn/lxc,hallyn/lxc,hallyn/lxc,caglar10ur/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,caglar10ur/lxc