package/ffmpeg: Fix mips compile

buildroot has no support for msa, CONFIG_CPU_HAS_MSA is not enabled in
kernel config.

Fixes
http://autobuild.buildroot.net/results/030/03055aa696e9e885baabb43fff613d751974a4b7//
http://autobuild.buildroot.net/results/6b5/6b5c13c949e8ae23f1b58c6da4002371130e9bb3//
http://autobuild.buildroot.net/results/1af/1af3c68bc21503438fb0066f5e51e82e35a5442b//


and many others

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>