Fix #ifdef check for older openssl versions

Small compile fix for older versions of openssl that do not have SHA256
support and hence do not define OPENSSL_NO_SHA256.

src/crypto/tls_openssl.c

@@ -716,7 +716,7 @@ void * tls_init(const struct tls_config *conf)
 #endif /* CONFIG_FIPS */
 		SSL_load_error_strings();
 		SSL_library_init();
-#ifndef OPENSSL_NO_SHA256
+#if (OPENSSL_VERSION_NUMBER >= 0x0090800fL) && !defined(OPENSSL_NO_SHA256)
 		EVP_add_digest(EVP_sha256());
 #endif /* OPENSSL_NO_SHA256 */
 		/* TODO: if /dev/urandom is available, PRNG is seeded