dco linux: avoid redefining ovpn enums (2.6)

dco linux: avoid redefining ovpn enums (2.6)

Starting with Linux kernel version 6.16, a couple of ovpn-related enum
definitions were introduced in the `include/uapi/linux/if_link.h`
header. Redefining them in openvpn when they are already present in the
system headers can lead to conflicts or build issues.

This commit ensures that enum redefinitions are avoided by conditionally
using the existing definitions from the system header when available.

This is the port to release/2.6 based on commit
1d3c2b67a73a0aa011c13e62f876d24e49d41df0.

Change-Id: I41c5dfc7489352a9534ff6b1585a5a81e0623ab1
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Antonio Quartulli <antonio@mandelbit.com>
Message-Id: <20250801130302.372311-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32470.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>