hwpmc: Don't wrap entire files in #ifdef DEV_ACPI.

A subsequent commit will instead use existing infrastructure to
exclude the files from hwpmc.ko for non-ACPI builds. Note that the
original commit left the files as optional in sys/conf/files.arm64.

This reverts commit 751d88119fdc1dff71d1390c7768942a9b857bc6.

Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D38736