sys/wait.h: add P_PIDFD from linux v5.4

allows waiting on a pidfd, in the future it might allow retrieving the
exit status by a non-parent process, see

  linux commit 3695eae5fee0605f316fbaad0b9e3de791d7dfaf
  pidfd: add P_PIDFD to waitid()

include/sys/wait.h

@@ -13,7 +13,8 @@ extern "C" {
 typedef enum {
 	P_ALL = 0,
 	P_PID = 1,
-	P_PGID = 2
+	P_PGID = 2,
+	P_PIDFD = 3
 } idtype_t;
 
 pid_t wait (int *);