Update .gitattributes to properly classify png files as binary.

[asfimport]

I'm working on trying to resurrect the parquet merge tool, and at least on my host it is trying to replace crlf in png which is breaking the merge tool.

Reporter: Micah Kornfield / @emkornfield
Assignee: Micah Kornfield / @emkornfield

PRs and other links:

• GitHub Pull Request #1372

_{Note: This issue was originally created as PARQUET-2494. Please see the migration documentation for further details.}