sssctl: Fix missing declaration
    
    The WEXITSTATUS is defined in stdlib.h on linux.
    There is a nice comment in stdlib.h:
      /* Define the macros <sys/wait.h> also would define this way.  */
    
    It's better to not rely on this and use more platfom friendly
    way with including "sys/wait.h". For example the libc on FreeBSD
    does not provide WEXITSTATUS in stdlib.h.
    
    I found this macro mentioned only in the manual page for wait(2)
    and there is mentioned just the "sys/wait.h" and not "stdlib.h"
    
    src/tools/sssctl/sssctl.c: In function 'sssctl_run_command':
    src/tools/sssctl/sssctl.c:110: error: implicit declaration of function
    'WEXITSTATUS'
    gmake[2]: *** [Makefile:22383: src/tools/sssctl/sssctl-sssctl.o] Error 1
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>