multimedia/avidemux: Disable -Werror=attributes and unbreak build with GCC 9
In file included from .../.build/ffmpeg/source/libavutil/common.h:437,
from .../.build/ffmpeg/source/libavutil/avutil.h:296, from .../.build/ffmpeg/source/libavutil/samplefmt.h:24, from .../.build/ffmpeg/source/libavcodec/avcodec.h:31, from .../avidemux_2.6.11/avidemux_core/ADM_core/src/ADM_memcpy.cpp:41:
.../.build/ffmpeg/source/libavutil/mem.h:185:79: error: 'alloc_size' attribute ignored on a function returning 'int' [-Werror=attributes]
185 | av_alloc_size(2, 3) int av_reallocp_array(void *ptr, size_t nmemb, size_t size);
| ^cc1plus: some warnings being treated as errors
PR: 238330