Add ChangeLog entry for #992 fix

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 9ee82c6..4240c85 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,10 @@
 
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+Bugfix
+   * Remove unused headers included in x509.c. Found by Chris Hanson and fixed
+     by Brendan Shanks. Part of a fix for #992.
+
 Security
    * Fix a bug in the X.509 module potentially leading to a buffer overread
      during CRT verification or to invalid or omitted checks for certificate