core/manager: silence gcc warning about unitialized variable

core/manager: silence gcc warning about unitialized variable

At -O3, this was printed a hundred times for various callers of
manager_add_job_by_name(). AFAICT, there is no error and `unit` is always
intialized. Nevertheless, add explicit initialization to silence the noise.

src/core/manager.c: In function 'manager_start_target':
src/core/manager.c:1413:16: warning: 'unit' may be used uninitialized in this function [-Wmaybe-uninitialized]
         return manager_add_job(m, type, unit, mode, e, ret);
                ^
src/core/manager.c:1401:15: note: 'unit' was declared here
         Unit *unit;
               ^