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

Include appropriate header file internal.h in proc.c because function
acpi_sleep_proc_init() has its prototype declaration in internal.h.

This eliminates the following warning in proc.c:
drivers/acpi/proc.c:148:12: warning: no previous prototype for ‘acpi_sleep_proc_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>