Add Makefile to .gitignore
Add Makefile to .gitignore

This allows devs to use custom Makefiles without cluttering the shared
codebase.

Closes chain/chainprv#100.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain