Fix the problem that frame rate becomes slower than the specified value.

In my environment(OSX and oF0.8.1), app's fps becomes slower than specified value by ofSetFrameRate().
For example, if I set fps to 60, the actual value becomes about 57.
Since oF0.8.0, time to sleep for adjust desired fps is calculated by micro seconds order, but it is slightly larger value. It may be caused by overhead of sleep function call, I don't know exactly.

I modified code to take into account entire time to call sleep function.
It doesn't  tested on other OS.
I appreciate your feedback!