lttng: clean-up: silence bogus string truncation warning

gcc 9.1.0 reports
enable_events.c:827:2: warning: ‘strncpy’ output truncated copying 9 bytes from a string of length 11 [-Wstringop-truncation]
  827 |  strncpy(ret, preamble, length);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

gcc seems confused and using "sizeof" the preamble seems to make
it understand the code flow and not generate the warning anymore.

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