.gitattributes: Treat SVG as binary

This may not be fully accurate. But changes to an SVG file are usually
better viewed with a graphical tool than a `diff` and contributor
stats are really messed up with svg files being treated as text files.

Author: maribu

.gitattributes

@@ -6,3 +6,4 @@ Cargo.lock binary
 # when the heading is exactly 7 characters long.
 *.md conflict-marker-size=100
 *.txt conflict-marker-size=100
+*.svg binary