x11/xpra: unbreak with ffmpeg 4.0

xpra/codecs/dec_avcodec2/decoder.c:5656:72: error: use of undeclared identifier 'CODEC_FLAG2_FAST'

__pyx_v_self->codec_ctx->flags2 = (__pyx_v_self->codec_ctx->flags2 | CODEC_FLAG2_FAST);
                                                                     ^

PR: 227726
Obtained from: upstream