add missing header for isdigit() in example parser

example/parser.cpp

@@ -4,6 +4,7 @@
 //    (See accompanying file LICENSE_1_0.txt or copy at
 //          http://www.boost.org/LICENSE_1_0.txt)
 
+#include <cctype>
 #include <cstdio>
 #include <exception>
 #include <functional>