ACPI: AGDI: Fix missing prototype warning for acpi_agdi_init()

When building with W=1, we get the following warning:

drivers/acpi/arm64/agdi.c:88:13: warning: no previous prototype for ‘acpi_agdi_init’ [-Wmissing-prototypes]
 void __init acpi_agdi_init(void)

Include AGDI driver's header file to pull in the prototype definition
for acpi_agdi_init() to get rid of the compiler warning

Fixes: a2a591fb ("ACPI: AGDI: Add driver for Arm Generic Diagnostic Dump and Reset device")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>