usr.sbin/pkg: Don't re-define roundup2

The file already includes sys/param.h and should use that definition.
I found this while testing D28332.

Reviewed By: bapt
Differential Revision: https://reviews.freebsd.org/D28331

(cherry picked from commit 9a0a48b12d3d85f0f00b99439e6a8779f5f1abb6)