time-util: define TIMESPEC_OMIT

Follow-up for 34c3d57474.

src/basic/time-util.h

@@ -75,6 +75,8 @@ typedef enum TimestampStyle {
 #define DUAL_TIMESTAMP_INFINITY ((dual_timestamp) { USEC_INFINITY, USEC_INFINITY })
 #define TRIPLE_TIMESTAMP_NULL ((triple_timestamp) {})
 
+#define TIMESPEC_OMIT ((const struct timespec) { .tv_nsec = UTIME_OMIT })
+
 usec_t now(clockid_t clock);
 nsec_t now_nsec(clockid_t clock);