Add some includes in C++ header files

Add some includes in C++ header files

Add some includes to fix errors shown by clangd when editing header
files.  These don't fix compilation errors.

Change-Id: I1792b50c5808d9a9c3f045decea97745aac6a77f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11496
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>