Fix build on NetBSD: Include <stdarg.h> for va_arg(3)

Fix build on NetBSD: Include <stdarg.h> for va_arg(3)

STDARG(3)                  Library Functions Manual                  STDARG(3)

NAME
     stdarg, va_arg, va_copy, va_end, va_start - variable argument lists

SYNOPSIS
     #include <stdarg.h>

     void
     va_start(va_list ap, last);

     type
     va_arg(va_list ap, type);

     void
     va_copy(va_list dest, va_list src);

     void
     va_end(va_list ap);

NetBSD 7.99                      June 14, 2015                     NetBSD 7.99

Commit migrated from https://github.com/dotnet/coreclr/commit/35c78b974239034a21d58fcad357f3d7b10c5ff4