Fix unused variable with MBEDTLS_SHA512_NO_SHA384

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 168735b..ff77d7d39 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1450,7 +1450,8 @@
                                    int minor_ver,
                                    mbedtls_md_type_t hash )
 {
-#if !defined(MBEDTLS_SSL_PROTO_TLS1_2) || !defined(MBEDTLS_SHA512_C)
+#if !defined(MBEDTLS_SSL_PROTO_TLS1_2) ||       \
+    !( defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_SHA512_NO_SHA384) )
     (void) hash;
 #endif