Fix compilation error with Mingw32

Fix compilation error on Mingw32 when `_TRUNCATE` is defined. Use
`_TRUNCATE` only if `__MINGW32__` not defined. Fix suggested by
Thomas Glanzmann and Nick Wilson on issue #355
diff --git a/ChangeLog b/ChangeLog
index 227faed..f8dcae5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+   * Fix compilation error on Mingw32 when `_TRUNCATE` is defined. Use `_TRUNCATE`
+     only if `__MINGW32__` not defined. Fix suggested by Thomas Glanzmann and
+     Nick Wilson on issue #355
+
 = mbed TLS 2.6.0 branch released 2017-08-10
 
 Security