Fixed the simple application build.

pkgs/applications/simple/default.nix

-{ stdenv, fetchgit, zeromq, intel, gsl, libnrm}:
+{ stdenv, fetchgit, zeromq, intel, gsl, libnrm, mpich2}:
 stdenv.mkDerivation {
   name = "simple";
   src = fetchgit {
@@ -6,7 +6,7 @@ stdenv.mkDerivation {
     sha256="0wpxb25bfry116rzjb8c66fpxqibrn1dby4xpfsbrnvbhhirck9z";
   };
 
-  buildInputs = [ zeromq intel.mkl intel.WrappedICC gsl libnrm ];
+  buildInputs = [ zeromq intel.mkl intel.WrappedICC gsl libnrm mpich2 ];
 
   buildPhase = "cd simple; make randomwalk";