package/mplayer: disable external mmx

package/mplayer: disable external mmx

Fixes a follow-up error with this defconfig
http://autobuild.buildroot.net/results/642/6422adeef19ec547c7bc3f8ad3b0d51702015240/
which occurs after disabling inline mmx with the previous patch:

This is the error message:

fmpeg/libavcodec/libavcodec.a(apedec.o): In function `ape_decode_frame':
apedec.c:(.text+0x1df5): undefined reference to `avpriv_emms_yasm'
ffmpeg/libavcodec/libavcodec.a(asvdec.o): In function `decode_frame':
asvdec.c:(.text+0x77c): undefined reference to `avpriv_emms_yasm'
ffmpeg/libavcodec/libavcodec.a(bink.o): In function `decode_frame':
bink.c:(.text+0x2809): undefined reference to `avpriv_emms_yasm'
ffmpeg/libavcodec/libavcodec.a(dvdec.o): In function `dvvideo_decode_frame':
dvdec.c:(.text+0x575): undefined reference to `avpriv_emms_yasm'
ffmpeg/libavcodec/libavcodec.a(ffv1dec.o): In function `decode_slice':
ffv1dec.c:(.text+0x3110): undefined reference to `avpriv_emms_yasm'
ffmpeg/libavcodec/libavcodec.a(h264.o):h264.c:(.text+0xea8): more undefined references to `avpriv_emms_yasm' follow

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