Not every user of idctdsp.h wants to initialize an IDCTDSPContext;
e.g. the proresdsp only uses ff_init_scantable_permutation()
and the IDCT permutation enum; similarly for cavsdsp and wmv2dsp.
Using a forward declaration here avoids an avcodec.h dependency
in the relevant files.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>