xbase: make linux-only

pkgs/development/libraries/xbase/default.nix

···
   meta = {
     homepage = http://linux.techass.com/projects/xdb/;
     description = "C++ class library formerly known as XDB";
-    platforms = stdenv.lib.platforms.all;
+    platforms = stdenv.lib.platforms.linux;
     maintainers = [ ];
   };
 }