continue cleaning up include file lists

src/fix_indent.cpp

@@ -15,9 +15,9 @@
    Contributing author: Ravi Agrawal (Northwestern U)
 ------------------------------------------------------------------------- */
 
+#include <mpi.h>
 #include <cmath>
 #include <cstring>
-#include <cstdlib>
 #include "fix_indent.h"
 #include "atom.h"
 #include "input.h"
@@ -26,7 +26,6 @@
 #include "lattice.h"
 #include "update.h"
 #include "modify.h"
-#include "output.h"
 #include "respa.h"
 #include "error.h"
 #include "force.h"