Bump session.xsd version to 2.12

The session schema has changed to include new tracked process
attributes. This change is technically non-backward compatible if
tracked PIDs were saved.

As it is extremely unlikely that anyone saves PIDs to a session
configuration to load it in another lttng-tools version, the schema
does not allow the pre-2.12 PID tracking specification to be
expressed. In my opinion, this is unlikely enough not to warrant a
major version bump.

This can be changed if someone really encounters this in the wild or
has a legitimate use case for going through this trouble.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I8fd8dac02065b09c3e3c75b38699caea3223ce50
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>