loader: Make pkcs5v2 unconditionally available

This was previously only available if GELI support was included, but I
want to use it for processing entropy from EFI

Reviewed by: imp
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46634

(cherry picked from commit 925f15731e3480fe80f580a2f7463cb5210a95c8)