BUILD: makefile/da: also clean Os/ in Device Atlas dummy lib dir

Commit b81483cf2 ("MEDIUM: da: update doc and build for new scheduler
mode service.") added a new directory to the Device Atlas dummy lib,
but this one is not cleaned during "make clean", causing build failures
sometimes when switching between compiler versions during development.

This should be backported to 2.6.
diff --git a/Makefile b/Makefile
index 2501cf3..47004ba 100644
--- a/Makefile
+++ b/Makefile
@@ -1078,6 +1078,7 @@
 	$(Q)rm -f addons/promex/*.[oas]
 	$(Q)rm -f addons/51degrees/*.[oas] addons/51degrees/dummy/*.[oas] addons/51degrees/dummy/*/*.[oas]
 	$(Q)rm -f addons/deviceatlas/*.[oas] addons/deviceatlas/dummy/*.[oas] addons/deviceatlas/dummy/*.o
+	$(Q)rm -f addons/deviceatlas/dummy/Os/*.o
 	$(Q)rm -f addons/ot/src/*.[oas]
 	$(Q)rm -f addons/wurfl/*.[oas] addons/wurfl/dummy/*.[oas]
 	$(Q)rm -f admin/*/*.[oas] admin/*/*/*.[oas]