fbcode_builder should be synced to the mcrouter repo

In https://github.com/facebook/folly/pull/2268, I added `fbcode_builder` manifests for https://github.com/facebook/mcrouter to facilitate adding a CMake build system on top of it in https://github.com/facebook/mcrouter/pull/449/. Now that the PR has been merged, the changes have been propagated to several repositories that use `fbcode_builder`, but not `mcrouter`.

Could `mcrouter` be setup with `fbcode_builder` as well like those repos?

Thanks in advance!