build: clean up finch-core _output directory in clean target (#290)

Issue #, if available:
#262
*Description of changes:*

*Testing done:*



- [x] I've reviewed the guidance in CONTRIBUTING.md


#### License Acceptance

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Signed-off-by: Anqi Pang <anqipans@amazon.com>

Makefile

@@ -283,6 +283,7 @@ mdlint-ctr:
 .PHONY: clean
 clean:
 	-@rm -rf $(OUTDIR) 2>/dev/null || true
+	-@rm -rf ./deps/finch-core/_output || true
 	-@rm ./*.tar.gz 2>/dev/null || true
 	-@rm ./*.qcow2 2>/dev/null || true
 	-@rm ./test-coverage.* 2>/dev/null || true