hurd: Do not include full <stdarg.h> in <hurd.h>

This fixes a hurd/check-installed-headers-c failure with
-std=c89 #define _FORTIFY_SOURCE 1:

In file included from ../hurd/hurd.h:354,
                 from ../sysdeps/hurd/include/hurd.h:2,
                 from /tmp/cih_test_9IaUwa.c:10:
/home/bmg/install/compilers/i686-gnu/lib/gcc/i686-glibc-gnu/13.2.1/include/stdarg.h:54:34: error: "__STDC_VERSION__" is not defined, evaluates to 0 [-Werror=undef]
   54 | #if !defined(__STRICT_ANSI__) || __STDC_VERSION__ + 0 >= 199900L \
      |                                  ^~~~~~~~~~~~~~~~
/home/bmg/install/compilers/i686-gnu/lib/gcc/i686-glibc-gnu/13.2.1/include/stdarg.h:55:8: error: "__cplusplus" is not defined, evaluates to 0 [-Werror=undef]
   55 |     || __cplusplus + 0 >= 201103L
      |        ^~~~~~~~~~~
cc1: all warnings being treated as errors

Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>