SOLARIS: Include sys/varargs.h for kernel stdarg

SOLARIS: Include sys/varargs.h for kernel stdarg

When compiling for the kernel, pre-10 Solaris seems to want
<sys/varargs.h> instead of <stdarg.h>, as documented in the Solaris
man pages. So include <sys/varargs.h> instead of <stdarg.h> where we
need variable arguments (afs_warn.c).

Reviewed-on: http://gerrit.openafs.org/3653
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 6d03eef7217be8f0b72f34ab023e69c976dea84d)

Change-Id: I473bcaa368c739265b911e79d596193f75ff8697
Reviewed-on: http://gerrit.openafs.org/3655
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>