Fix empty union when TLS is disabled

When all TLS 1.2 support is disabled, union mbedtls_ssl_premaster_secret was
empty, which is not valid C even if the union is never used. Fixes #6628.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 68e8d04..006b255 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -620,6 +620,7 @@
 
 /* Dummy type used only for its size */
 union mbedtls_ssl_premaster_secret {
+    unsigned char dummy; /* Make the union non-empty even with SSL disabled */
 #if defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED)
     unsigned char _pms_rsa[48];                         /* RFC 5246 8.1.1 */
 #endif