The new droplet compatible storage backend introduced in #1790 adds two new dependencies, that were not added to the debian control files. This commit adds the appropriate packages as build dependencies.
Without this cmake will try to load the dependencies by itself and will fail, because git is not available by default during the package build.

Please note: If you are building for bullseye, you will either need a backport of libcli11-dev and libmsgsl-dev or you will need to have git available during the packaging.

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