Fix sxiv .desktop file (#648)

Would previously error out as such:

Opening "<file>" with Image viewer  (image/jpeg)
sxiv: file://<filepath>
sxiv: No valid image file given, aborting

.local/share/applications/img.desktop

@@ -1,4 +1,4 @@
 [Desktop Entry]
 Type=Application
 Name=Image viewer
-Exec=/usr/bin/sxiv -a %u
+Exec=/usr/bin/sxiv -a %f