www/wordpress: Fix "call to undefined function filter_var()"

WordPress needs USE_PHP=filter installed, otherwise some things will
fail complaining about "call to undefined function filter_var()".

PR: 278330
Approved by: joneum (maintainer, timeout > 1.5 years)
MFH: 2025Q4

(cherry picked from commit ba7d478eb1112d20d73dd430d9d513330fbae4fa)