ff_ps_init() initializes some tables for AAC parametric stereo
and some of them are only valid for the fixed- or floating-point
decoder, whereas others (namely VLCs) are valid for both.
The latter are therefore initialized by ff_ps_init_common()
and because the two versions of ff_ps_init() can be run
concurrently, it is guarded by an AVOnce.

Yet now that there is ff_aacdec_common_init_once() there is
a better way to do this: Call ff_ps_init_common()
from ff_aacdec_common_init_once(). That way there is no need
to guard ff_ps_init_common() by an AVOnce any more.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>