blame: provide type of fingerprints pointer

blame: provide type of fingerprints pointer

The fingerprints member of struct blame_origin is a void pointer that is
only ever used to reference objects of type struct fingerprint.  Declare
its type to allow the compiler to do type checks.  We can keep its type
opaque in blame.h, though -- only functions in blame.c need to know the
actual definition of struct fingerprint.

Signed-off-by: René Scharfe <redacted>
Reviewed-by: Barret Rhoden <redacted>
Signed-off-by: Junio C Hamano <redacted>