Close warning in opus_encoder.c while enable fuzzing

[willson-chen]

stereo_voice_threshold, stereo_music_threhold and mode_threshold, these var only used in fuzzing.

[willson-chen]

There are two warnings left.

celt/rate.c:251:82: warning: unused parameter ‘prev’ [-Wunused-parameter]
       int *ebits, int *fine_priority, int C, int LM, ec_ctx *ec, int encode, int prev, int signalBandwidth)
                                                                                  ^~~~
celt/rate.c:251:92: warning: unused parameter ‘signalBandwidth’ [-Wunused-parameter]
       int *ebits, int *fine_priority, int C, int LM, ec_ctx *ec, int encode, int prev, int signalBandwidth)
                                                                                            ^~~~~~~~~~~~~~~

These two input var can't be excluded unless modifing the function declaration.

[jmvalin]

Seems to be fixed. Please re-open if not