declare incomplete type struct itimerspec in timerfd.h

normally time.h would provide a definition for this struct, but
depending on the feature test macros in use, it may not be exposed,
leading to warnings when it's used in the function prototypes.

include/sys/timerfd.h

@@ -13,6 +13,8 @@ extern "C" {
 
 #define TFD_TIMER_ABSTIME 1
 
+struct itimerspec;
+
 int timerfd_create(int, int);
 int timerfd_settime(int, int, const struct itimerspec *, struct itimerspec *);
 int timerfd_gettime(int, struct itimerspec *);