Add rubyarchhdrhdir to include path

The current generated makefile couldn't find `ruby/config.h` on my system because the appropriate directory wasn't being added to the include flags. `RbConfig::CONFIG["arch_hdrdir"]` returns `nil` and the `archdirs` built below didn't generate the necessary path.

This is on x86_64 Ubuntu 13.10 with ruby 2.0.0p299.
The missing path was `/usr/include/x86_64-linux-gnu/ruby-2.0.0/`