[3.2] initialize slice_directory::index_header::version to avoid warning in tests

[spoonincode]

The lack of an initializer for version was leading to a use before set warning in the trace_api_plugin tests

leap/plugins/trace_api_plugin/test/test_trace_file.cpp

Lines 436 to 437 in 631b3bf

	slice_directory::index_header h;
	const auto data = fc::raw::pack(h);

Our general practice is to initialize PODs in structs, so let's do that here.