PNP: ACPI: Fix missing-prototypes in acpi_pnp.c

Fix the following warnings.
drivers/acpi/acpi_pnp.c:382:13: warning: no previous prototype
for ‘acpi_pnp_init’ [-Wmissing-prototypes].

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
[ rjw: Subject edit ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>