Fix: invalid alignment of enumeration fields

Issue
---

According to the CTF specification, the alignment of an enumeration is
that of its container integer field declaration. However, an
enumeration field's alignment is always initialized to 1.

This causes babeltrace to fail to read traces produced by lttng-ust
following a fix that causes it to generate extended event headers. The
problem is observed on ARM platforms since lttng-ust will produce a
layout that does not result in unaligned memory accesses.

Solution
---

The alignment of the enumeration declaration's container is sampled
when the enumeration declaration is created.

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