mkcomposefs: Fix error message

[eriksjolund]

failed_path is declared here:

composefs/tools/mkcomposefs.c

Line 1508 in 14ad6b9

char *failed_path;

It looks like failed_path has not been set when it's used here:

composefs/tools/mkcomposefs.c

Line 1672 in 14ad6b9

err(EXIT_FAILURE, "error computing digest %s", failed_path);

An alternative fix is to use src_path.
I just took the easiest alternative and removed the argument, but I can change that if you want.