Use of sparse_array.c only in the shared libssl

Use of sparse_array.c only in the shared libssl

Conditioning it on $disabled{shared} isn't right, it will still end up
in the static variant of the library.  It's better to use SHARED_SOURCE
for these sorts of things.

Fixes #20238

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20240)