avauddec: Use proper version guard
Just like we do with avviddec
../subprojects/gst-libav/ext/libav/gstavauddec.c: In function ‘gst_ffmpegauddec_frame’:
../subprojects/gst-libav/ext/libav/gstavauddec.c:596:3: error: ‘frame_number’ is deprecated [-Werror=deprecated-declarations]
596 | ffmpegdec->context->frame_number++;
| ^~~~~~~~~