security/gnupg: rebuild man pages on every build

gnupg ships its man pages as texinfo files, precompiled into info files.
This change causes make to rebuild them every time. There are two reasons:

• Rebuilding them automatically corrects several Linuxisms in paths (e.g. /etc => /usr/local/etc).

• Rebuilding them is a neccessary precondition for making any local changes that will affect the content of the man pages, which I intend to do in a future commit.

Reviewed by: adamw
Approved by: adamw (maintainer)
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D22471