Fix: add mmap_base_offset to ctf_stream_pos

When dealing with mmap traces, the whole trace buffer is mmapped in
babeltrace and we may need to read it one subbuffer at a time.
In order to do that, we need to keep a reference of the base of the
current subbuffer inside the mmap region.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>