update comment, maybe just libroken dependencies: or so, such that it doesn't get out of date again

It might be good to return the exact value of errno as set by the call to atexit(3) in case of failures. According to its manual page and to its source code, ENOMEM is the only likely one, but atexit(3) from the libc is a weak symbol and might be overridden.

From what I can tell OSSL_PROVIDER_load(3) simply returns OK/FAIL and is not expected to set errno to any specific value, so EINVAL is probably as good as it gets.

Rest of SRCS is nicely sorted (as it should be), either slot this into the right spot,
or if you want to call it out as an addition use SRCS+=

you can remove the extra line