hwmon: (acpi_power_meter) fix lockdep spew due to non-static lock class

Similar to a30dcb4f which fixed asus_atk0110.ko, I recently received a
bug report from someone hitting the same issue in acpi_power_meter.

[   13.963168] power_meter ACPI000D:00: Found ACPI power meter.
[   13.963900] BUG: key ffff8802161f3920 not in .data!
[   13.963904] ------------[ cut here ]------------
[   13.963915] WARNING: at kernel/lockdep.c:2986
	lockdep_init_map+0x52f/0x560()

So let's fix that up for them by statically declaring the
lockdep_class_key.

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Cc: stable@vger.kernel.org # 3.0+
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>