unix: add support for timerfd syscalls on Linux

timerfd_create, timerfd_gettime and timerfd_settime syscalls have been added.
Fixes golang/go#38733

Change-Id: I306d68103b6efb2515c74f384646210c4b68f66e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/230798
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/linux/types.go

@@ -46,6 +46,7 @@ package unix
 #include <sys/statvfs.h>
 #include <sys/sysinfo.h>
 #include <sys/time.h>
+#include <sys/timerfd.h>
 #include <sys/times.h>
 #include <sys/timex.h>
 #include <sys/un.h>
@@ -410,6 +411,8 @@ type Timeval C.struct_timeval
 
 type Timex C.struct_timex
 
+type ItimerSpec C.struct_itimerspec
+
 const (
 	TIME_OK    = C.TIME_OK
 	TIME_INS   = C.TIME_INS