Squelches the following compiler warnings:

libavcodec/opusenc.c:1051:16: warning: format specifies type 'long' but
the argument has type 'long long' [-Wformat]
               avctx->bit_rate/1000, clipped_rate/1000);
               ^~~~~~~~~~~~~~~~~~~~
libavcodec/opusenc.c:1051:38: warning: format specifies type 'long' but
the argument has type 'long long' [-Wformat]
               avctx->bit_rate/1000, clipped_rate/1000);
                                     ^~~~~~~~~~~~~~~~~