security/openfortivpn: Fix build with openssl from ports

Typo in CFLAGS: ${OPENSSLINC} => -I${OPENSSLINC}

PR: 287912
Approved by: Atanu Biswas <atanubiswas484@gmail.com> (maintainer, implicit - fix port)
MFH: 2025Q3

(cherry picked from commit 302ba4b88cdbe0a05a18f653921cddc77f18ad1f)