ACPI: fix acpi_debugfs_init prototype

acpi_debugfs_init function is declared with return type int in
drivers/acpi/internal.h when CONFIG_DEBUG_FS is enabled, but its
definition in drivers/acpi/debugfs.c has return type void. This is due
to commit aecad432 ("ACPI: Cleanup custom_method debug stuff"),
which changed the return type from int to void without updating the
declaration.

Fix this inconsistency by updating acpi_debugfs_init prototype.  While
at it, include internal.h in debugfs.c so that the compiler can check
that the declaration and definition remain compatible.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>