change CLOCK_REALTIME to CLOCK_MONOTONIC because it is more robust

[julianstoerig]

The fenster_time function should use CLOCK_MONOTONIC instead of CLOCK_REALTIME as the argument to clock_gettime, since this is "not affected by discontinuous in the system time (e.g., if the system manually changes the clock)[...] All CLOCK_MONOTONIC variants guarantee that the time returned by consecutive calls will not go backwards". (man page)

This is in contrast to the currently used CLOCK_REALTIME, which is affected by such jumps.