binary_writer.hpp:869: (style) Consider using std::accumulate algorithm instead of a raw loop.

https://github.com/Xav83/nlohmann-json-cppcheck/commit/910a7d2b873dd7ae92ec81cced2bf73200ff4848/checks#step:5:107Signed-off-by: Xav83 <x.jouvenot@gmail.com>