Set *.png and *.ico to be treated as binary files.

The change b732beba makes all files text files and sets lf as eol. This
will automatically change all files to have lf if they are touched by
git (e.g. via git status). This change cannot be stashed and makes it
hard to work with the repo (rebase and checkout don't really work). See
also #6183.

Here, we set the offending files (*.png and *.ico, but that might be
more in the future) to be treated as binary files and not be changed by
git.

.gitattributes

 llm/ext_server/* linguist-vendored
 * text eol=lf
+*.png binary
+*.ico binary