replace C-style include files with their C++ equivalents

src/finish.cpp

@@ -12,10 +12,10 @@
 ------------------------------------------------------------------------- */
 
 #include <mpi.h>
-#include <math.h>
-#include <stdlib.h>
-#include <string.h>
-#include <stdio.h>
+#include <cmath>
+#include <cstdlib>
+#include <cstring>
+#include <cstdio>
 #include "finish.h"
 #include "timer.h"
 #include "universe.h"