Add missing bit to ChangeLog entry for the latest update

in gcc/ada/gcc-interface/trans.c.

From-SVN: r221392

gcc/ada/ChangeLog

@@ -3,6 +3,7 @@
 	* gcc-interface/trans.c (Attribute_to_gnu) <Code_Address case>:
 	On targets where a function symbol designates a function descriptor,
 	fetch the function code address from the descriptor.
+	(USE_RUNTIME_DESCRIPTORS): Provide a default definition.
 
 2015-03-04  Robert Dewar  <dewar@adacore.com>