Making openssl_tls_init_hook OpenSSL specific

Commit 896fcdb230e72 (sorry for chiming in too late, I missed that thread)
added a TLS init hook which is OpenSSL specific: openssl_tls_init_hook. Since
the rest of the TLS support in the backend is library agnostic, we should IMO
make this hook follow that pattern, else this will make a non-OpenSSL backend
not compile.

If we make the hook generic, extension authors must have a way to tell which
backend invoked it, so maybe the best option is to simply wrap this hook in
USE_OPENSSL ifdefs and keep the name/signature? Looking at the Secure
Transport patch I wrote, there is really no equivalent callsite; the same goes
for a libnss patch which I haven't yet submitted.

The attached adds USE_OPENSSL guards.

cheers ./daniel