QElapsedTimer on Linux can use system time instead of monitonic clock

Implementation of qt_clock_gettime() (qelapsedtimer_unix.cpp) depends on existing of definition of CLOCK_REALTIME. The cpp-file includes "time.h" and "sys/time.h". Those headers don't contain the definition (Ubuntu 16.04). The definition is included in /usr/include/linux/time.h and is not visible for the cpp-file. But time.h contains extern-declaration for clock_gettime() function. So the code will be compiled, but gettimeofday() will be used on Ubuntu (and maybe other distributives) instead of clock_gettime(CLOCK_MONOTONIC).