ffmpeg: fix neon support for aarch64

ffmpeg: fix neon support for aarch64

ffmpeg ported arm 32-bit NEON code to aarch64 as hard dependency:
http://ffmpeg.org/pipermail/ffmpeg-devel/2016-November/202733.html

Buildroot option BR2_ARM_CPU_HAS_NEON handles only the 32bit case:
https://git.buildroot.net/buildroot/commit/?id=0976cd6cd64a128a2ac921e4f35f0e7cbe306692

For aarch64 we need to make sure to pass --enable-neon in order to
prevent a link error, fixes

http://autobuild.buildroot.net/results/fa4/fa46003249664b1f97f6c18253c84cd3d6fb9cdb/

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