platform: Include stdarg.h where needed

Windows builds also need stdarg.h included when using variadic
functions.

Fixes #2656
diff --git a/ChangeLog b/ChangeLog
index fbc9d7b..e2389d5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -65,6 +65,8 @@
      returning. Fixes #2431.
    * Fix typo in net_would_block(). Fixes #528 reported by github-monoculture.
    * Remove redundant include file in timing.c. Fixes #2640 reported by irwir.
+   * Fix build failure when building with mingw on Windows by including
+     stdarg.h where needed. Fixes #2656.
 
 API Changes
    * Extend the MBEDTLS_SSL_EXPORT_KEYS to export the handshake randbytes,