src.ctf.fs: data-stream-file.cpp: declare variables on first use

src.ctf.fs: data-stream-file.cpp: declare variables on first use

Move variables declarations to be at or closer to their first use.

Change-Id: I0649c4fda3b42c6bca02217481c08b575fda6a5d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8402
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12345
Tested-by: jenkins <jenkins@lttng.org>