Shell: Wait for *any* child to change state when receiving a SIGCHLD

This really just works around the core issue, which is that we have no
reliable way to know exactly who raised the signal (yet).
Fixes #3645, in a very weird (yet apparently standard) way.

Shell/main.cpp

@@ -34,7 +34,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/wait.h>
 
 RefPtr<Line::Editor> editor;
 Shell* s_shell;