Ignore Brewfile.lock.json

Why is this change needed?
--------------------------
This appears to be a generated state file, particular to the device.

How does it address the issue?
------------------------------
Ignore the generated file.

Any links to any relevant tickets, articles, or other resources?
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Homebrew/homebrew-bundle#1188

Did you complete all of the following?
--------------------------------------
- Run test suite?
- Add new tests?
- Consider security implications and practices?

.gitignore

@@ -1 +1,2 @@
 .stow-local-ignore
+/Brewfile.lock.json