sysutils/openzfs-kmod: fix the build with a nonstandard SRC_BASE

ports that need FreeBSD's src code should check $SRC_BASE rather than assume
/usr/src. For example, it should be possible to build openzfs-kmod with a
command like this:

env SRC_BASE=$HOME/freebsd/base/head make

Approved by: freqlabs (maintainer)
MFH: 2020Q3
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D26033