Simplify "typedef struct dt_iop_params_t ..." to "typedef void dt_iop_params_t;"

FIXME: i moved typedef into pixelpipe.h to fix redefinition (or cyclic dependency). Find a better place.
Merge pull request #587 from LebedevRI/typedef-void-dt_iop_params_t

Simplify "typedef struct dt_iop_params_t ..." to "typedef void dt_iop_pa...