package/mplayer: fix aarch64 compilation

Added code to define HAVE_ARMV8 when needed.

Fixes
http://autobuild.buildroot.net/results/5f8/5f85c32eb89aac48ae8da892d9800bd13274cd3e/



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