efiapi.h: fix EventGroup parameter of EFI_CREATE_EVENT_EX prototype

From UEFI specifications, this parameter is a pointer, see
https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_final.pdf
page 148 for reference.

Signed-off-by: leo <leo.sartre@geebol.fr>

inc/efiapi.h

@@ -576,7 +576,7 @@ EFI_STATUS
     IN EFI_TPL                  NotifyTpl,
     IN EFI_EVENT_NOTIFY         NotifyFunction OPTIONAL,
     IN const VOID               *NotifyContext OPTIONAL,
-    IN const EFI_GUID           EventGroup OPTIONAL,
+    IN const EFI_GUID           *EventGroup OPTIONAL,
     OUT EFI_EVENT               *Event
     );