bdiff: include util.h

Without this, IS_PY3K isn't define and the preprocessor uses the
incorrect module loading code, causing the module fail to load at
run-time.

After this patch, all our C extensions (except for watchman's) appear
to import correctly in Python 3!

mercurial/bdiff_module.c

@@ -17,6 +17,7 b''
 
 #include "bdiff.h"
 #include "bitmanipulation.h"
+#include "util.h"
 
 
 static PyObject *blocks(PyObject *self, PyObject *args)