ACPI / dock: Include appropriate header file in dock.c

Includes appropriate header file internal.h in dock.c because
function acpi_dock_init() has its prototype declaration in
internal.h.

This eliminates the following warning in dock.c:
drivers/acpi/dock.c:899:13: warning: no previous prototype for ‘acpi_dock_init’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>