Add missing pragma once

This was found when running gcc -ftime-report, which at the end flags
this as possibly missing include header.

This appears to have been missed in
f51d863523b7bfec5c45b0a847178b5d853404d9 where this was initially added.
Add it.

Tested: code compiles

Change-Id: I31e0ea55c080c239edcd366627ce0829ef9ac41b
Signed-off-by: Ed Tanous <etanous@nvidia.com>
diff --git a/include/duplicatable_file_handle.hpp b/include/duplicatable_file_handle.hpp
index 5401e76..f199485 100644
--- a/include/duplicatable_file_handle.hpp
+++ b/include/duplicatable_file_handle.hpp
@@ -1,3 +1,5 @@
+#pragma once
+
 // SPDX-License-Identifier: Apache-2.0
 // SPDX-FileCopyrightText: Copyright OpenBMC Authors
 #include <unistd.h>