__MAC_10_11 can be present in updated revision of an older SDK so it
can't reliably detect availability of kAudioFormatEnhancedAC3 constant.

Fixes: b4daa2c4 ('lavc/audiotoolboxdec: add eac3 decoder')
Cc: Rodger Combs <rodger.combs@gmail.com>
Signed-off-by: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
Previous version reviewed by: Rodger Combs <rodger.combs@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>