BUG: Fix build on OSX: add missing 'close' function to Epoll

The Epoll.close function was recently added to Linux_ext, but it wasn't also
added to the "epoll unimplemented" module, so Core fails to build on OSX. This
adds the close function to the "unimplemented" module.

Signed-off-by: Mike McClurg <mike.mcclurg@gmail.com>