Fix loading the hwpmc module when ACPI is enabled

In 0a9a4d2cd6092 a check for OPT_ACPI was added to the hwpmc Makefile
to fix loading the module in a kernel where ACPI has been disabled.
This broke loading the module when ACPI was enabled in the build as
OPT_ACPI isn't a Makefile macro so was always disabled.

Move this check to the C files where the DEV_ACPI macro does exist.

Reviewed by: gnn
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D37773