Use absolute import path for c++.capnp

This allows {schema,rpc,rpc-twoparty}.capnp to be used without modification when
relocated outside of the C++ Cap'n Proto codebase.

I'm pretty sure this is safe -- I was able to `make check && make install` successfully after this change -- but I could be wrong.  If that's the case, I'll begrudgingly maintain a modified local copy of schema.capnp .  :)