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/ChangeLog.d/ssl_premaster_secret-empty.txt b/ChangeLog.d/ssl_premaster_secret-empty.txt
new file mode 100644
index 0000000..0ce5f36
--- /dev/null
+++ b/ChangeLog.d/ssl_premaster_secret-empty.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Fix a compilation error on some platforms when including mbedtls/ssl.h
+     with all TLS support disabled. Fixes #6628.