www/lexbor: install headers unconditionally
- Always install headers, remove HEADERS option
- Use dynamic SOVERSION via PLIST_SUB
PR: 283924
MFH: 2025Q1
Fixes: 97d9bb985218fbacf81ad48e08a811839ba7c50f
(cherry picked from commit e2bc8743fb2ec5b805a23a103de3c45349a54356)