avcodec/motion_est_template: replace qsort with AV_QSORT

This code is in a performance critical section. AV_QSORT is
substantially faster due to the inlining of the comparison callback.
Thus, the increase in performance is worth the increase in binary size.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>