Add druntime's rt directory to the list of directories to omit in the test/tools/sanitize_json.d

[JinShil]

This PR is in support of dlang/druntime#2634

That PR publicly imports a couple of modules from druntime's rt directory into object.d. It causes the test https://github.com/dlang/dmd/blob/master/test/compilable/json2.d to fail because it adds those modules to the json output, but the sanitize_json.d tool has not been programmed to eliminate the rt directory like it has for std, core, and etc. See https://auto-tester.puremagic.com/show-run.ghtml?projectid=1&runid=3659831&isPull=true

This PR adds rt to the list of directories to omit in the santitize_json.d tool.

This PR is required for dlang/druntime#2634 to pass the test suite.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#9992"