avoid preprocessor warnings, by placing Python.h include file on the top, as suggested by python docs

src/PYTHON/fix_python.cpp

@@ -11,6 +11,7 @@
    See the README file in the top-level LAMMPS directory.
 ------------------------------------------------------------------------- */
 
+#include <Python.h>
 #include <stdio.h>
 #include <string.h>
 #include "fix_python.h"