Fix: automatic enum value overwrites existing error code value

Fix: automatic enum value overwrites existing error code value

Automatic enum value are generated using the last value and
incrementing it. Having backward compat assignation before the
LTTNG_ERR_NR would end up overwriting the error string since the
LTTNG_ERR_NR value would be LTTNG_ERR_PID_NOT_TRACKED + 1.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I4076866a28961fa49eed018e8249d2cd72979a84
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>