This allows to merge it with AACDecDSP.init and remove the latter
(it is called only once anyway); it also allows to make
the fixed/float AACDecDSP and AACDecProc implementations internal
to aacdec_fixed/float.c (which also fixes a violation of our
naming conventions). And it some linker errors when either decoder
is disabled.
Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>