security/openvpn: Fix detection of native inotify on FreeBSD 15

This fixes build when ASYNC_PUSH option is enabled

Fixed upstream in commit 38243844f225517fa1d288517db9a34a33c5ad13

Reference:
https://github.com/OpenVPN/openvpn/commit/38243844f225517fa1d288517db9a34a33c5ad13

PR: 293176
Reported by: Ivo Karabojkov
Approved by: mandree@ (via Matrix)