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

src/MISC/fix_viscosity.cpp

@@ -16,10 +16,10 @@
                         for swapping atoms of different masses
 ------------------------------------------------------------------------- */
 
-#include <math.h>
+#include <cmath>
 #include <mpi.h>
-#include <string.h>
-#include <stdlib.h>
+#include <cstring>
+#include <cstdlib>
 #include "fix_viscosity.h"
 #include "atom.h"
 #include "domain.h"