sxiv: installs .desktop file

sxiv won't appear in meus as it has NoDisplay but it can be used to
generate the mimetypes.

pkgs/applications/graphics/sxiv/default.nix

@@ -23,6 +23,11 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ libX11 imlib2 giflib libexif ];
 
+  postInstall = ''
+    mkdir -p $out/share/applications/
+    cp -v sxiv.desktop $out/share/applications/
+  '';
+
   meta = {
     description = "Simple X Image Viewer";
     homepage = https://github.com/muennich/sxiv;