Include <cstddef> in python modules to workaround Swig bug.

* wrap/python/spot.i, wrap/python/buddy.i: Include <cstddef>
because Swig 2.0.2 uses ptrdiff_t and does not do the include
itself.  In g++ most libstdc++ standard headers have been changed
to no longer include <cstddef> as an implementation detail, so
the difference shows.

wrap/python/spot.i

@@ -21,6 +21,12 @@
 // Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
 // 02111-1307, USA.
 
+%{
+  // Workaround for SWIG 2.0.2 using ptrdiff_t but not including cstddef.
+  // It matters with g++ 4.6.
+#include <cstddef>
+%}
+
 %module(directors="1") spot
 
 %include "std_string.i"