Fixed compilation of MPI tests without MPI

tests/mpi/mpiio-save-load-test.cpp

@@ -99,6 +99,6 @@ int main(int argc, char **argv)
 
 #else
 
-main(){}
-#endif
+int main(){}
 
+#endif

tests/mpi/mpiio-save-test.h

@@ -88,6 +88,6 @@ int main(int argc, char **argv)
 
 #else
 
-main(){}
+int main(){}
 
 #endif