nginx: Set sa_handler to wrapped signal handler

The nginx demo was previously not using the signal handler we defined so this
fixes that. Before we were rewriting source files in-place, sa_handler was set
to ngx_timer_signal_handler (i.e. the unwrapped version) by the git patch. After
we switched to writing sources to an output directory, I copied what the patch
was doing by adding an IA2_IGNORE but it should have been set to the signal
handler we defined.

external/nginx/src/event/ngx_event.c

@@ -684,7 +684,7 @@ ngx_event_process_init(ngx_cycle_t *cycle)
         struct itimerval  itv;
 
         ngx_memzero(&sa, sizeof(struct sigaction));
-        sa.sa_handler = IA2_IGNORE(ngx_timer_signal_handler);
+        sa.sa_handler = IA2_SIGHANDLER(ngx_timer_signal_handler);
         sigemptyset(&sa.sa_mask);
 
         if (sigaction(SIGALRM, &sa, NULL) == -1) {