Silence strncpy warning emitted by GCC 8 in lttng_strncpy()

GCC 8 warns if the destination's length is passed to strncpy,
since that could cause the destination to not be NULL-terminated.

This is not a concern for the lttng_strncpy since it checks
that the source must not be truncated. Therefore, it is safe
to simply use strcpy().

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