Fix pure cpp1 header file generated using -import-std

[MatthieuHernandez]

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I've noticed that if I execute cppfront.exe with the -import-std option on a .h file that doesn't contain any cpp2 code, then an #endif will be missing at the end of the file. See https://github.com/hsutter/cppfront/issues/1320 for more info.

⚠️ I haven't added any test because I didn't see any tests with .h files.