ff_huff_build_tree uses qsort underneath. AV_QSORT is substantially
faster due to the inlining of the comparison callback. Furthermore, this
code is reasonably performance critical, since in e.g the fraps codec,
ff_huff_build_tree is called on every frame. This routine is also called
in vp6 on every frame in some circumstances.

Sample benchmark (x86-64, Haswell, GNU/Linux), vp6 from FATE:
vp6 (old):
  78930 decicycles in qsort,       1 runs,      0 skips
  45330 decicycles in qsort,       2 runs,      0 skips
  27825 decicycles in qsort,       4 runs,      0 skips
  17471 decicycles in qsort,       8 runs,      0 skips
  12296 decicycles in qsort,      16 runs,      0 skips
   9554 decicycles in qsort,      32 runs,      0 skips
   8404 decicycles in qsort,      64 runs,      0 skips
   7405 decicycles in qsort,     128 runs,      0 skips
   6740 decicycles in qsort,     256 runs,      0 skips
   7540 decicycles in qsort,     512 runs,      0 skips
   9498 decicycles in qsort,    1024 runs,      0 skips
   9938 decicycles in qsort,    2048 runs,      0 skips
   8043 decicycles in qsort,    4095 runs,      1 skips

vp6 (new):
  15880 decicycles in qsort,       1 runs,      0 skips
  10730 decicycles in qsort,       2 runs,      0 skips
  10155 decicycles in qsort,       4 runs,      0 skips
   7805 decicycles in qsort,       8 runs,      0 skips
   6883 decicycles in qsort,      16 runs,      0 skips
   6305 decicycles in qsort,      32 runs,      0 skips
   5854 decicycles in qsort,      64 runs,      0 skips
   5152 decicycles in qsort,     128 runs,      0 skips
   4452 decicycles in qsort,     256 runs,      0 skips
   4161 decicycles in qsort,     511 runs,      1 skips
   4081 decicycles in qsort,    1023 runs,      1 skips
   4072 decicycles in qsort,    2047 runs,      1 skips
   4004 decicycles in qsort,    4095 runs,      1 skips
Reviewed-by: Timothy Gu <timothygu99@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>