ffmpeg: disable freetype support on microblaze/glibc

Fixes http://autobuild.buildroot.net/results/6dd/6ddefaf001b9275ee073effbd6eb07be9f43e24d/



While it does provide fenv.h, it doesn't support the needed exceptions so
disable it.

Reported-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>