Remove out/args.gn when cleaning.

This way, after cleaning, the platform build config will be copied
afresh so a different platform can be targeted.

Change-Id: Iaea422b871479bf0c790f7314af56ba74066dec9
diff --git a/Makefile b/Makefile
index 8ad1562..9a2cf40 100644
--- a/Makefile
+++ b/Makefile
@@ -32,6 +32,7 @@
 .PHONY: clean
 clean:
 	@$(NINJA) -C $(OUT) -t clean
+	rm -f $(OUT)/args.gn
 
 .PHONY: clobber
 clobber: