ignore images line ending on older git versions

On Ubuntu 16.04 git 2.7.4 tries to fix the line ending of .png and .ico
files, and obviously it ruins them. This commit adds an attribute to
those files to properly mark them as binary.

Author: pietroalbini

.gitattributes

@@ -9,3 +9,7 @@ src/etc/installer/gfx/* binary
 *.woff binary
 src/vendor/** -text
 Cargo.lock -merge linguist-generated=false
+
+# Older git versions try to fix line endings on images, this prevents it.
+*.png binary
+*.ico binary