Add stdarg.h to fix issues with va_start

Otherwise I got errors like these:

http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/sys-apps/shadow/files/shadow-4.1.5-stdarg.patch?view=diff&r1=text&tr1=1.1&r2=text&tr2=1.1&diff_format=s