tools: ynl: fix the header guard name for OVPN

Thorsten reports that after upgrading system headers from linux-next
the YNL build breaks. I typo'ed the header guard, _H is missing.

Reported-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/59ba7a94-17b9-485f-aa6d-14e4f01a7a39@leemhuis.info


Fixes: 12b19656 ("tools: ynl: add missing header deps")
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250423220231.1035931-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>