include: add IWYU pragmas in private header files

include: add IWYU pragmas in private header files

Add Include What You Use "private" pragmas in the private header files
exported by babeltrace 2, indicating that <babeltrace2/babeltrace.h>
should be used instead.

Change-Id: I7b7bdde4ab52012b4b57d5c7739e7910a5321f9c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11491
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>