This allows to inline it in af_afir.c (regardless of interposing);
moreover it removes a dependency of the checkasm test on
lavfi/af_afir.o.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Only the AudioFIRDSPContext and the functions for its initialization
are needed outside of lavfi/af_afir.c.
Also rename the header to af_afirdsp.h to reflect the change.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>