add scipy/interpolate/interpnd.pyx.in to source tarball (scipy#1780)

MANIFEST.in

@@ -9,7 +9,7 @@ include setupegg.py
 include setup.py
 include scipy/*.py
 # Add Cython files
-recursive-include scipy *.pyx *.pxd *.pxi
+recursive-include scipy *.pyx *.pyx.in *.pxd *.pxi
 # Adding scons build related files not found by distutils
 recursive-include scipy SConstruct SConscript
 recursive-include scipy README