Ignore compiler-generated files

[jhadida]

All in the title; I simply added a few lines in .gitignore to ignore compiler-generated files.

Ignoring these object files should prevent future issues for contributors who may otherwise accidentally include them in PRs.

It may also be worth including a note for contributors to run:

git update-index --assume-unchanged util/lbfgsb/Makefile

again to avoid including local changes to that file in PRs.

[alshedivat]

Thanks!