libprocstat: ZFS support: Makefile: Tidy up a bit

Regroup assignments tweaking preprocessor defines/undefs, and separately
those about include directories.

Re-order include directories a bit more logically, and remove redundant
ones.

Separate logical groups by blank lines.

Build artifacts have been verified to stay the same when produced with
an external LLVM 18 toolchain.

MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48070