[gitignore] add //tmp to gitignore

This path is usefully near the top of the tree, but is extremely useful to
have a nearby stashing ground for transitive work. It is not for use by
builds or tools, just for human produced data, such as trace files and the
like.

Test: n/a
Change-Id: I3010fc75dfa384664dad4c5994fc20d8af92032d