Suppress -Wdangling-pointer in event_signal_closure()

Suppress -Wdangling-pointer in event_signal_closure()

gcc 12 complains:

    [34/46] Building C object CMakeFiles/event_static.dir/event.c.o
    /src/le/libevent/event.c: In function ‘event_signal_closure’:
    /src/le/libevent/event.c:1384:32: warning: storing the address of local variable ‘ncalls’ in ‘*ev.ev_.ev_signal.ev_pncalls’ [-Wdangling-pointer=]
     1384 |                 ev->ev_pncalls = &ncalls;
          |                 ~~~~~~~~~~~~~~~^~~~~~~~~
    /src/le/libevent/event.c:1378:15: note: ‘ncalls’ declared here
     1378 |         short ncalls;
          |               ^~~~~~
    /src/le/libevent/event.c:1378:15: note: ‘ev’ declared here