little change with the latest libav

need a little change if using the latest libav (git clone
git://github.com/libav/libav.git libav)
fix errors while compiling libvpvl2 with bullet2.80

the error log:
16:21:07: Running build steps for project libvpvl2...
16:21:07: Starting: "/usr/bin/make"
Scanning dependencies of target vpvl2
[  4%] Building CXX object
CMakeFiles/vpvl2.dir/src/core/vmd/MorphAnimation.cc.o
In file included from
/Users/ooyama/Downloads/MMDAI/libvpvl2/src/core/vmd/MorphAnimation.cc:39
:
In file included from
/Users/ooyama/Downloads/MMDAI/libvpvl2/include/vpvl2/vpvl2.h:40:
In file included from
/Users/ooyama/Downloads/MMDAI/libvpvl2/include/vpvl2/Common.h:66:
/Users/ooyama/Downloads/MMDAI/bullet/src/LinearMath/btAlignedObjectArray
.h:328:12: error: no matching function for call to object of type
'const vpvl2::vmd::MorphAnimationKeyFramePredication'
while (CompareFunc(m_data[i],x))
^~~~~~~~~~~
/Users/ooyama/Downloads/MMDAI/bullet/src/LinearMath/btAlignedObjectArray
.h:353:5: note: in instantiation of function template specialization
'btAlignedObjectArray<vpvl2::vmd::MorphKeyframe
*>::quickSortInternal<vpvl2::vmd::MorphAnimationKeyFramePredication>'
requested here

quickSortInternal(CompareFunc,0,size()-1);
^
/Users/ooyama/Downloads/MMDAI/libvpvl2/include/vpvl2/Common.h:159:18:
note: in instantiation of function template specialization
'btAlignedObjectArray<vpvl2::vmd::MorphKeyframe
*>::quickSort<vpvl2::vmd::MorphAnimationKeyFramePredication>' requested
here
m_values.quickSort(comparable);
^
/Users/ooyama/Downloads/MMDAI/libvpvl2/src/core/vmd/MorphAnimation.cc:15
2:16: note: in instantiation of function template specialization
'vpvl2::Array<vpvl2::vmd::MorphKeyframe
*>::sort<vpvl2::vmd::MorphAnimationKeyFramePredication>' requested here
frames.sort(MorphAnimationKeyFramePredication());
^
/Users/ooyama/Downloads/MMDAI/libvpvl2/src/core/vmd/MorphAnimation.cc:68
:10: note: candidate function not viable: 'this' argument has type
'const vpvl2::vmd::MorphAnimationKeyFramePredication', but method is
not marked const
bool operator()(const BaseKeyframe *left, const BaseKeyframe
*right) {
^
In file included from
/Users/ooyama/Downloads/MMDAI/libvpvl2/src/core/vmd/MorphAnimation.cc:39
:
In file included from
/Users/ooyama/Downloads/MMDAI/libvpvl2/include/vpvl2/vpvl2.h:40:
In file included from
/Users/ooyama/Downloads/MMDAI/libvpvl2/include/vpvl2/Common.h:66:
/Users/ooyama/Downloads/MMDAI/bullet/src/LinearMath/btAlignedObjectArray
.h:330:12: error: no matching function for call to object of type
'const vpvl2::vmd::MorphAnimationKeyFramePredication'
while (CompareFunc(x,m_data[j]))
^~~~~~~~~~~
/Users/ooyama/Downloads/MMDAI/libvpvl2/src/core/vmd/MorphAnimation.cc:68
:10: note: candidate function not viable: 'this' argument has type
'const vpvl2::vmd::MorphAnimationKeyFramePredication', but method is
not marked const
bool operator()(const BaseKeyframe *left, const BaseKeyframe
*right) {
^
2 errors generated.
make[2]: *** [CMakeFiles/vpvl2.dir/src/core/vmd/MorphAnimation.cc.o]
Error 1
make[1]: *** [CMakeFiles/vpvl2.dir/all] Error 2
make: *** [all] Error 2
16:21:07: The process "/usr/bin/make" exited with code 2.
Error while building project libvpvl2 (target: Desktop)
When executing build step 'Make'