warning: typedef ‘Char_type’ locally defined but not used

[kkoenen]

Warning during build;

In file included from src/bitcoinrpc.h:16:0,
                 from src/init.cpp:11:
src/json/json_spirit_writer_template.h: In function ‘String_type json_spirit::non_printable_to_string(unsigned int)’:
src/json/json_spirit_writer_template.h:31:50: warning: typedef ‘Char_type’ locally defined but not used [-Wunused-local-typedefs]
         typedef typename String_type::value_type Char_type;
                                                  ^~~~~~~~~