systemctl: fix compiler warning in list_timers()

get_next_elapse() will always fill 'next' with values when it
returns >= 0. Hence, the compiler is wrong about this warning.
Initialize 'next' nevertheless.

src/systemctl/systemctl.c: In function ‘list_timers’:
src/systemctl/systemctl.c:953:43: warning: ‘next.monotonic’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
                                 converted = nw.realtime - (nw.monotonic - next.monotonic);
                                           ^
In file included from ./src/shared/log.h:30:0,
                 from src/systemctl/systemctl.c:46:
./src/shared/macro.h:137:38: warning: ‘next.realtime’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
                         _a < _b ? _a : _b;      \
                                      ^
src/systemctl/systemctl.c:933:32: note: ‘next.realtime’ was declared here
                 dual_timestamp next;
                                ^