Fix: compat: 'LTTNG_UST_ABI_PROCNAME_LEN' is undeclared

Fix: compat: 'LTTNG_UST_ABI_PROCNAME_LEN' is undeclared

Observed issue
==============

On old systems, the `lttng_pthread_setname_np` function fallsback to
using the compat prctl version. In that context,
`LTTNG_UST_ABI_PROCNAME_LEN` is indeed not declared.

Solution
========

Use `LTTNG_PTHREAD_NAMELEN`. This mimics what is done in other versions
of `lttng_pthread_setname_np`.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I73956cacd7b1e9400881b17b1cd89db2530d3a00