Fix mbedtls_ssl_get_version() for TLSv1.3

Test it in ssl-opt.sh.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 436e15c..adb18ab 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2241,7 +2241,8 @@
     {
         case MBEDTLS_SSL_MINOR_VERSION_3:
             return( "TLSv1.2" );
-
+        case MBEDTLS_SSL_MINOR_VERSION_4:
+            return( "TLSv1.3" );
         default:
             return( "unknown" );
     }