Add some additional files to .gitignore (#823)
Add some additional files to .gitignore (#823)

- I've had an ubuntu logfile showing up in my git, so ignore all `.log`
  files
- `Makefile` and `extconf.h` are auto-generated based on the user's
  environment, so ignoring those as well.
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick