ACPI: bus: add missing forward declaration to acpi_bus.h

The header references struct notifier_block but neither includes
linux/notifier.h nor contains the relevant forward declaration.

Add the latter for correctness.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
[ rjw: Subject tweak ]
Link: https://patch.msgid.link/20260427112238.132419-1-bartosz.golaszewski@oss.qualcomm.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: brgl

include/acpi/acpi_bus.h

@@ -17,6 +17,8 @@
 #include <linux/property.h>
 #include <linux/types.h>
 
+struct notifier_block;
+
 struct acpi_handle_list {
 	u32 count;
 	acpi_handle *handles;