allcodecs: move libaacplus down.

We choose the first encoder by default and libaccplus has a
quite limited set of supported bitrates/sample rates.
Thus leading to failure by default in many cases when it is
enabled at compile time.
Moving it down means that the other aac encoders are favored
by default which avoids this issue.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
