add ifdef to allow compilation without mpi

git-svn-id: https://ppm.inf.ethz.ch/svn/ppmnumerics/branches/ngtopo/libppmnumerics@1029 7c7fe9aa-52eb-4d9e-b0a8-ba7d787348e9

src/poisson/ppm_poisson_solve.f

@@ -23,8 +23,9 @@
       USE ppm_module_map
 
       IMPLICIT NONE
+#ifdef __MPI
       include 'mpif.h'
-
+#endif
       !-------------------------------------------------------------------------
       ! Arguments
       !-------------------------------------------------------------------------