Silence useless warning in src/json/json_spirit_writer_template.h to make important warnings easier to see.

warning: typedef ‘Char_type’ locally defined but not used [-Wunused-local-typedefs]
Merge pull request #2980 from wtogami/silenceunusedwarning

Silence useless warning in src/json/json_spirit_writer_template.h to mak...