Mark `*.tza` files as binary in `.gitattributes` for old Git versions

This prevents `thirdparty/oidn/weights/rtlightmap_hdr.tza` from always being
considered as modified when using an old Git version (such as the one on
Ubuntu 16.04).

.gitattributes

@@ -13,3 +13,4 @@ thirdparty/* linguist-vendored
 *.jar binary
 *.png binary
 *.ttf binary
+*.tza binary