package/python-jsonschema: add missing runtime dependency

python-attrs package is required since version 2.6.0.

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

package/python-jsonschema/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_PYTHON_JSONSCHEMA
 	bool "python-jsonschema"
+	select BR2_PACKAGE_PYTHON_ATTRS # runtime
 	select BR2_PACKAGE_PYTHON_PYRSISTENT # runtime
 	help
 	  An implementation of JSON Schema validation for Python.