Include sys/wait.h to avoid compiler warning

gcc warned about an implicit declaration of function 'wait3'. 
Including this header fixes this.

src/sentinel.c

@@ -36,6 +36,7 @@
 #include <ctype.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>
+#include <sys/wait.h>
 
 extern char **environ;