Add changelog entry for the zlib support fix

Signed-off-by: Simon Butcher <simon.butcher@arm.com>
diff --git a/ChangeLog b/ChangeLog
index 82483e8..6ba9a5a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,10 @@
 
 = mbed TLS "baremetal" branch
 
+Bugfix
+   * Fix build failure with MBEDTLS_ZLIB_SUPPORT enabled. Reported by
+     Jack Lloyd in #2859. Fix submitted by jiblime in #2963.
+
 Features
    * Add new configuration option MBEDTLS_SSL_NO_SESSION_CACHE that enables
      code size savings in configurations where cache-based session resumption is