Use loop_control.loop_var directly

6eefc11c converted task.loop_control into an object, but while the other
callers were updated to use .loop_var instead of .get('loop_var'), this
site was overlooked.

This can be reproduced by including with loop_control a file that does
set_fact; a simple regression test along these lines is included.

test/integration/test_includes.yml

@@ -1 +1,3 @@
 - include: test_includes2.yml parameter1=asdf parameter2=jkl
+
+- include: test_includes3.yml