Remove duplicate "const" qualifier for *matrix.

This silences out a warning when compiling with Intel C Compiler.
Patch by Diego 'Flameeyes' Pettenò %flameeyes A gmail P com%

Originally committed as revision 15524 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/audioconvert.c

@@ -77,7 +77,7 @@ struct AVAudioConvert {
 
 AVAudioConvert *av_audio_convert_alloc(enum SampleFormat out_fmt, int out_channels,
                                        enum SampleFormat in_fmt, int in_channels,
-                                       const const float *matrix, int flags)
+                                       const float *matrix, int flags)
 {
     AVAudioConvert *ctx;
     if (in_channels!=out_channels)