avformat/hlsenc: correctly compute target duration

With HLS, the duration of all segments must be lower or equal to the target
duration. Therefore floor(duration + 0.5) yields incorrect results.

For example, for duration = 1.35, floor(duration + 0.5) yields 1.0, but the
correct result is 2.0.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>