Add changelog entry for the zlib support fix
mbed TLS ChangeLog (Sorted per branch, date)
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+ * Fix build failure with MBEDTLS_ZLIB_SUPPORT enabled. Reported by
+ Jack Lloyd in #2859. Fix submitted by jiblime in #2963.
+
= mbed TLS 2.20.0 branch released 2020-01-15
Bugfix
