Add cwchar to format.h for std::fputws

[concatime]

• I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

While trying to build fmtlib with EASTL, I realized that cwchar was missing:

include/fmt/format.h:3964:7: error: no member named 'fputws' in namespace 'std'; did you mean simply 'fputws'?
  if (std::fputws(buffer.data(), f) == -1)

[vitaut]

Thank you!

[concatime]

Always a pleasure :)