MAINT Include binary_tree.pxi in source distribution

Fixes scikit-learn#3331.

MANIFEST.in

@@ -1,7 +1,7 @@
 include *.rst
 recursive-include doc *
 recursive-include examples *
-recursive-include sklearn *.c *.h *.pyx *.pxd
+recursive-include sklearn *.c *.h *.pyx *.pxd *.pxi
 recursive-include sklearn/datasets *.csv *.csv.gz *.rst *.jpg *.txt
 include COPYING
 include AUTHORS.rst