security/vuxml: Fix make vuln-flat.xml

According to the documentation, we should be able to type:

make vuln-flat.xml

(https://docs.freebsd.org/en/books/porters-handbook/book/#security-notify-vuxml-testing)

But after 87748de634d7b the target name includes the PKGDIR which is not right.

Fixes: 87748de634d7b