fenics: restrict to Darwin

It has never been building on Linux.

pkgs/development/libraries/science/math/fenics/default.nix

···
   meta = {
     description = "The FEniCS Problem Solving Environment in Python and C++";
     homepage = http://fenicsproject.org/;
-    platforms = stdenv.lib.platforms.all;
+    platforms = stdenv.lib.platforms.darwin;
     license = stdenv.lib.licenses.lgpl3;
   };
 }