Update sysdeps/unix/sysv/linux/ifaddrs.c

Fix

../sysdeps/unix/sysv/linux/ifaddrs.c:835:27: error: ‘getifaddrs’ aliased to undefined symbol ‘__getifaddrs’
 weak_alias (__getifaddrs, getifaddrs)
                           ^
../sysdeps/unix/sysv/linux/ifaddrs.c:844:28: error: ‘freeifaddrs’ aliased to undefined symbol ‘__freeifaddrs’
 weak_alias (__freeifaddrs, freeifaddrs)

caused by

commit 4009ddc6
Author: Samuel Thibault <samuel.thibault@ens-lyon.org>
Date:   Thu Sep 28 00:19:18 2017 +0200

    hurd: Fix `getifaddrs' and `freeifaddrs' symbol exposition

	* sysdeps/unix/sysv/linux/ifaddrs.c (__getifaddrs): Add
	libc_hidden_def.
	(__freeifaddrs): Likewise.