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 273d4bc..8667c9e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,10 @@
 
 = mbed TLS 2.18.1 branch released 2019-07-12
 
+Bugfix
+   * Fix build failure when building with mingw on Windows by including
+     stdarg.h where needed. Fixes #2656.
+
 Changes
    * Enable building of Mbed TLS as a CMake subproject. Suggested and fixed by
      Ashley Duncan in #2609.