Fix duplicate mutex allocation in threads_win.c

Fix duplicate mutex allocation in threads_win.c

Creating an rcu lock does a double allocation of the underlying mutex.
Not sure how asan didn't catch this, but we clearly have a duplicate
line here

Fixes #24085

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24086)