exit-status: add missing include for SIG* defines

Otherwise building fails with glibc 2.16. It works with glibc >= 2.17
because it is implicitly included via macro.h -> sys/param.h -> signal.h

src/basic/exit-status.c

@@ -20,6 +20,7 @@
 ***/
 
 #include <stdlib.h>
+#include <signal.h>
 
 #include "exit-status.h"
 #include "set.h"