Don't skip the current provider in ossl_provider_register_child_cb

Don't skip the current provider in ossl_provider_register_child_cb

This restriction was in place to avoid problems with recursive attempts
to aquire the flag lock/store lock from within a provider's init function.
Since those locks are no longer held when calling the init function there
is no reason for the restriction any more.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15854)