Wrong preproccessor condition fix

Fix for issue #696
Change #if defined(MBEDTLS_THREADING_PTHREAD)
to #if defined(MBEDTLS_THREADING_C)
diff --git a/ChangeLog b/ChangeLog
index 9034b42..7a72030 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,6 +5,9 @@
 Bugfix
    * Add a check if iv_len is zero, and return an error if it is zero. reported
      by roberto. #716
+   * Replace preproccessor condition from #if defined(MBEDTLS_THREADING_PTHREAD)
+     to #if defined(MBEDTLS_THREADING_C) as the library cannot assume they will
+     always be implemented by pthread support. Fix for #696
 
 Security
    * Fix authentication bypass in SSL/TLS: when auth_mode is set to optional,