ACPI: battery: make function __battery_hook_unregister() static

The function __battery_hook_unregister is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
drivers/acpi/battery.c:654:6: warning: symbol '__battery_hook_unregister'
was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>