avcodec/libfdk_aacenc: enable 7.1 channel encoding

7.1(wide) and 7.1(wide-side) channel layouts are supported in fdk_aac since october 2013 (commit fa3eba1644)

Signed-off-by: Jean First <jeanfirst@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>