signal/sig_kill.c: remove the limitation of kill(0, xx)

since there are situation which send singal to idle thread already,

CONFIG_SCHED_CHILD_STATUS=y
CONFIG_SCHED_HAVE_PARENT=y

Signo SIGCHLD will send to parent group, when child exit

Change-Id: Iceb2ac41948c1c3418839a3b5de70985d48c75d1
Signed-off-by: ligd <liguiding1@xiaomi.com>

sched/signal/sig_kill.c

@@ -81,7 +81,7 @@ int nxsig_kill(pid_t pid, int signo)
 
   /* We do not support sending signals to process groups */
 
-  if (pid <= 0)
+  if (pid < 0)
     {
       return -ENOSYS;
     }