Backend specific ifdefs in sha2.h

The #ifdef guards in sha2.h are using USE_SSL when they in fact are guarding
the inclusion of OpenSSL specific code.  This has never caused any issues as
there only is a single supported TLS backend in core so far, but since we’ve
spent a significant amount of energy on making the TLS backend non-hardcoded
it seems we should fix this too.  The Makefile around sha2.c/sha2_openssl.c is
already testing for openssl rather than ssl (which given src/Makefile.global
variables makes perfect sense of course).

cheers ./daniel