Add ChangeLog entry for redundant mutex initialization optimizations
mbed TLS ChangeLog (Sorted per branch, date)
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Changes
+ * Do not define global mutexes around readdir() and gmtime() in
+ configurations where the feature is disabled. Found and fixed by Gergely
+ Budai.
+
= mbed TLS 2.7.x branch released 2018-xx-xx
Default behavior changes
