Fix error in elpa_index
lfind() expects a proper comparison function where the order of the arguments does not matter. Previously it was assumed that the first argument is always a pointer to an unspecific "key" object and the second argument a pointer to an array member to compare against. However, the standard requires the two arguments to be interchangeable pointers to array-member like objects. Surfaced as musl-libc does in fact call it the other way around, compared to glibc.