kill(): fix bug when running with POSIX extension but without pcntl

SIGKILL constant is defined by pcntl, not by posix. If pcntl is not compiled then bugs can occur when trying to kill() the server (such as during a crash).

src/pocketmine/PocketMine.php

@@ -377,7 +377,7 @@ namespace pocketmine {
 			case "linux":
 			default:
 				if(function_exists("posix_kill")){
-					posix_kill($pid, SIGKILL);
+					posix_kill($pid, 9); //SIGKILL
 				}else{
 					exec("kill -9 " . ((int) $pid) . " > /dev/null 2>&1");
 				}