add autotools-generated files to .gitignore

When running "`make check`", Automake generates `test-suite.log`, along with various `*.log` and `*.trs` files in the tree. This pull request adds these files to `.gitignore`.

(It looks like this feature arrived in Automake 1.13.)

For a sample failure, see http://gitbuilder.sepia.ceph.com/gitbuilder-ceph-tarball-saucy-amd64-basic/log.cgi?log=0afa5530730db6a83f60b1ff9fffd41e926249e0

Resolves [#7052](http://tracker.ceph.com/issues/7052)