timing: Remove redundant include file

There is no need to include winbase.h, as it will be pulled in by
windows.h as needed.

Fixes #2640
diff --git a/ChangeLog b/ChangeLog
index 04570e4..fbc9d7b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -64,6 +64,7 @@
      codes to always wrap these codes into X.509 high level error codes before
      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.
 
 API Changes
    * Extend the MBEDTLS_SSL_EXPORT_KEYS to export the handshake randbytes,