Fix: get encoding for char arrays and sequences

The bt_ctf_get_encoding works for strings and integer, but not for char
arrays or sequences. For these compound types, we need to return the
encoding of the integer contained inside. This patch adds the support of
char arrays and sequences.

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