Fix winstore provider to work with recent decoder changes

Fix winstore provider to work with recent decoder changes

Changes made recently in commit 31b5f3f made changes to how a default
decoder was created, in which ossl_decoder_instance_new() started
returning null.  Other storemgmt providers were updated to start using
ossl_decoder_instance_new_forprov, but the winstore manager seems to
have got missed.  Fix it up properly

Fixes #27355

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27384)