Add missing forward declaration to resolve compile issue

xtldr/modules/xtos_o/includes/xtos.h

@@ -49,6 +49,10 @@ XTCDECL
 LOADER_MEMORY_TYPE
 XtConvertEfiMemoryType(IN EFI_MEMORY_TYPE EfiMemoryType);
 
+XTCDECL
+ULONG
+XtpDeterminePagingLevel(IN CONST PWCHAR Parameters);
+
 XTCDECL
 EFI_STATUS
 XtEnablePaging(IN PXTBL_PAGE_MAPPING PageMap);