Explicitly mark mbedtls_ssl_session.exported as private

This was an oversight during concurrent merging
in the run-up to Mbed TLS 3.0.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 3a14a58..e69bdb6 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -925,7 +925,7 @@
     unsigned char MBEDTLS_PRIVATE(id)[32];       /*!< session identifier */
     unsigned char MBEDTLS_PRIVATE(master)[48];   /*!< the master secret  */
 
-    unsigned char exported;
+    unsigned char MBEDTLS_PRIVATE(exported);
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 #if defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)