Renaming the decoder to speedhqdec.c makes sense since
we have an encoder in speedhqenc.c. Given that ff_rl_speedhq
is also used by the encoder, it is kept in speedhq.c
and a proper header for it is created to replace the ad-hoc
declaration in speedhqenc.c. This also allows to remove
the check for CONFIG_SPEEDHQ_DECODER, as it is always true
when speedhqdec.c is compiled.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>