src: fix windows build

Apparently MSVS compiles src/iconv.cc and deps/libiconv/iconv.c to the
same iconv.obj - in other words, one overwrites the other. The build
itself still succeeds but the DLL won't load because it's missing
symbols. Fix that by renaming src/iconv.cc to src/binding.cc.

Hat tip to Shigeki Ohtsu.

Fixes #62.