Make sure the pointless dune_solvers program continues to compile

[[Imported from SVN: r12925]]

src/dune_solvers.cc

@@ -2,7 +2,7 @@
 # include "config.h"     
 #endif
 #include <iostream>
-#include"dune/common/mpihelper.hh" // An initializer of MPI
+#include <dune/common/parallel/mpihelper.hh> // An initializer of MPI
 #include"dune/common/exceptions.hh" // We use exceptions
 
 int main(int argc, char** argv)