package/ffmpeg: Fix i586 compilation with gcc5

In file included from libavcodec/cabac_functions.h:46:0,
                 from libavcodec/h264_cabac.c:37:
libavcodec/h264_cabac.c: In function 'ff_h264_decode_mb_cabac':
libavcodec/x86/cabac.h:192:5: error: 'asm' operand has impossible constraints
     __asm__ volatile(

To reproduce the bug use this defconfig:

BR2_GCC_VERSION_5_1_X=y
BR2_PACKAGE_FFMPEG=y
BR2_PACKAGE_FFMPEG_GPL=y
BR2_PACKAGE_FFMPEG_NONFREE=y
BR2_PACKAGE_FFMPEG_FFPLAY=y
BR2_PACKAGE_FFMPEG_FFSERVER=y
BR2_PACKAGE_FFMPEG_FFPROBE=y
BR2_PACKAGE_FFMPEG_AVRESAMPLE=y
BR2_PACKAGE_FFMPEG_POSTPROC=y

[Thomas: add a comment in the code.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>