init: Make WaitToBeReaped() wait less long

Reduce the time spent in WaitToBeReaped() by waiting for SIGCHLD instead
of waiting for 50 ms.

Bug: 308687042
Change-Id: I5e259fdd22dec68e45d27205def2fc6463c06ca3
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/init/init.h b/init/init.h
index 9c7e918..b781167 100644
--- a/init/init.h
+++ b/init/init.h
@@ -28,6 +28,8 @@
 namespace android {
 namespace init {
 
+extern int sigchld_fd;
+
 Parser CreateParser(ActionManager& action_manager, ServiceList& service_list);
 Parser CreateApexConfigParser(ActionManager& action_manager, ServiceList& service_list);