We don't need to upstream this, because systemd 253 removes all signal stuff
from async.c.

--- elogind-252.9/src/basic/async.c.old	2023-05-15 06:35:29.000000000 -0500
+++ elogind-252.9/src/basic/async.c	2023-11-27 04:39:38.561566175 -0600
@@ -2,6 +2,7 @@
 
 #include <errno.h>
 #include <pthread.h>
+#include <signal.h>
 #include <stddef.h>
 #include <unistd.h>