Add missing forward reference for AcGetAcpiTable() routine

xtldr/modules/acpi/includes/acpi.h

@@ -18,6 +18,12 @@ XTCDECL
 EFI_STATUS
 AcGetAcpiDescriptionPointer(OUT PVOID *AcpiTable);
 
+XTCDECL
+EFI_STATUS
+AcGetAcpiTable(IN CONST UINT Signature,
+               IN PVOID PreviousTable,
+               OUT PVOID *AcpiTable);
+
 XTCDECL
 EFI_STATUS
 AcGetApicBase(OUT PVOID *ApicBase);