Move OPENSSL_add_builtin back into provider.c

Move OPENSSL_add_builtin back into provider.c

An earlier stage of the refactor in the last few commits moved this
function out of provider.c because it needed access to the provider
structure internals. The final version however no longer needs this so
it is moved back again.

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