lib/util: Fix build for lttng libraries that does not support tracef

lib/util: Fix build for lttng libraries that does not support tracef

On a system that has older lttng-ust development headers installed that
do not yet provide the tracef api, configure fails with:

Checking for library lttng-ust : yes
ERROR: Target 'lttng-ust' in directory /samba/lib/util re-defined as EMPTY - was SYSLIB

Fix the initialization order in waf, to not redefine the lttng-ust
target

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>