continue cleaning up include file lists

src/fix_restrain.cpp

@@ -16,9 +16,9 @@
      support for bond and angle restraints by Andres Jaramillo-Botero (Caltech)
 ------------------------------------------------------------------------- */
 
+#include <mpi.h>
 #include <cmath>
 #include <cstring>
-#include <cstdlib>
 #include "fix_restrain.h"
 #include "atom.h"
 #include "force.h"
@@ -26,7 +26,6 @@
 #include "domain.h"
 #include "comm.h"
 #include "respa.h"
-#include "input.h"
 #include "math_const.h"
 #include "memory.h"
 #include "error.h"