xmobar: add to all-packages so it shows up in search

People are confused when they "nix search" for xmobar (a standalone
application which just happens to be written in Haskell) and don't
find it; they assume xmobar is not actually packaged. They have to
have special Nix knowledge to know that they should look in
haskellPackages for xmobar.

Let's just put xmobar in all-packages.nix so it shows up in search,
and people can install it straightforwardly without knowing about
haskellPackages.