add TFD_TIMER_CANCEL_ON_SET that timerfd.h was missing

linux commit 575b1967e10a1f3038266244d2c7a3ca6b99fed8 moved timerfd
apis to a new uapi header which showed musl was missing this flag.

include/sys/timerfd.h

@@ -12,6 +12,7 @@ extern "C" {
 #define TFD_CLOEXEC O_CLOEXEC
 
 #define TFD_TIMER_ABSTIME 1
+#define TFD_TIMER_CANCEL_ON_SET (1 << 1)
 
 struct itimerspec;