Add .gitignore entry for .theia

Add .gitignore entry for .theia

This directory is used for project-specific settings for the Theia IDE.
Since I use that, I would like if the directory was ignored.  Even
though it doesn't apply to everybody, it shouldn't bother anybody to
have this entry.  Also, I wouldn't mind if we added similar entries for
other IDEs, if it helped other people.

Change-Id: Ic6f42b9d3f1d855e3d6e9e0dee1492aa484010ac
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1327
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>