Libopenh264enc would set the slice according to the number of cpu cores
if uiSliceNum equals to 0 (auto) in SM_FIXEDSLCNUM_SLICE mode.

Prompt a warning for user to catch this.
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Martin Storsjö <martin@martin.st>