gnustep.back: add runtime dependency on libXmu

Fix the failure of running applications like GWorkspace, which depends
on 'back'. It fails with a message similar to the one below:

Error (objc-load):/nix/store/fpxksxkl26qd5a7ay52mzv5qbj8di6b5-gnustep-back-0.25.0/lib/GNUstep/Bundles/libgnustep-back-025.bundle/./libgnustep-back-025: undefined symbol: XmuLookupStandardColormap

pkgs/desktops/gnustep/back/default.nix

···
 , x11
 , freetype
 , pkgconfig
+, libXmu
 }:
 let
   version = "0.25.0";
···
     url = "ftp://ftp.gnustep.org/pub/gnustep/core/gnustep-back-${version}.tar.gz";
     sha256 = "14gs1b32ahnihd7mwpjrws2b8hl11rl1wl24a7651d3z2l7f6xj2";
   };
-  buildInputs = [ cairo base gui freetype pkgconfig x11 ];
+  buildInputs = [ cairo base gui freetype pkgconfig x11 libXmu ];
   meta = {
     description = "A generic backend for GNUstep";
   };