sched.h: add CLONE_PIDFD from linux v5.2

when set a pidfd is stored in parent_tidptr, see

  linux commit b3e5838252665ee4cfa76b82bdf1198dca81e5be
  clone: add CLONE_PIDFD

include/sched.h

@@ -49,6 +49,7 @@ int     sched_yield(void);
 #define CLONE_FS	0x00000200
 #define CLONE_FILES	0x00000400
 #define CLONE_SIGHAND	0x00000800
+#define CLONE_PIDFD	0x00001000
 #define CLONE_PTRACE	0x00002000
 #define CLONE_VFORK	0x00004000
 #define CLONE_PARENT	0x00008000