sched.h: add CLONE_NEWTIME from linux v5.6

reuses a bit from CSIGNAL so it can only be used with unshare
and clone3, added in

  linux commit 769071ac9f20b6a447410c7eaa55d1a5233ef40c
  ns: Introduce Time Namespace

include/sched.h

@@ -49,6 +49,7 @@ int     sched_yield(void);
 
 #ifdef _GNU_SOURCE
 #define CSIGNAL		0x000000ff
+#define CLONE_NEWTIME	0x00000080
 #define CLONE_VM	0x00000100
 #define CLONE_FS	0x00000200
 #define CLONE_FILES	0x00000400