N3672 Editorial Fix

Added #include <initializer_list> to the <optional> synopsis, to be
consistent with other standard headers that use initializer_list.

Author: Stefanus Du Toit

source/utilities.tex

@@ -2053,6 +2053,8 @@
 
 \indexlibrary{\idxhdr{optional}}%
 \begin{codeblock}
+#include <initializer_list>
+
 namespace std {
   // \ref{optional.object}, optional for object types
   template <class T> class optional;