Makefile: Restore vendor directory after dep-validate

Currently, this target makes changes to the vendor directory, which may
overwrite local changes or cause build errors. It's better to restore
the original vendor directory after running the check.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

Makefile

@@ -95,4 +95,5 @@ dep-validate:
 	@$(VNDR)
 	@test -z "$$(diff -r vendor .vendor.bak 2>&1 | tee /dev/stderr)" || \
 		(echo >&2 "+ inconsistent dependencies! what you have in vendor.conf does not match with what you have in vendor" && false)
-	@rm -Rf .vendor.bak
+	@rm -Rf vendor
+	@mv .vendor.bak vendor