python-singledispatch: add missing runtime dependency

setup.py mentions python-six as required.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/python-singledispatch/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_PYTHON_SINGLEDISPATCH
 	bool "python-singledispatch"
 	depends on BR2_PACKAGE_PYTHON
+	select BR2_PACKAGE_PYTHON_SIX # runtime
 	help
 	  This library brings functools.singledispatch from
 	  Python 3.4 to Python 2.6-3.3