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

drivers/acpi/dock.c

@@ -33,6 +33,8 @@
 #include <linux/stddef.h>
 #include <linux/acpi.h>
 
+#include "internal.h"
+
 #define PREFIX "ACPI: "
 
 #define ACPI_DOCK_DRIVER_DESCRIPTION "ACPI Dock Station Driver"