Add python3-setuptools-scm to Build-Depends

Phase 2 of Spec 0003 will wire `linux-pkg`'s `packages/sdb/config.sh`
to derive `PACKAGE_VERSION` from `setuptools_scm` (running on the
build host, before dh-virtualenv enters the venv) so the .deb
metadata version tracks upstream sdb tags automatically and
`debian/changelog`'s upstream-version prefix never goes stale across
upstream syncs.

That host-side `python3 -m setuptools_scm` invocation needs the
package present on the build host. Stage the Build-Depends here so
the Phase 2 linux-pkg change is a single-file flip — and so any
manual local builds with the new config.sh don't fail with
"setuptools_scm not found" after `install_build_deps_from_control_file`.

The dependency is dormant in Phase 1 — nothing in `debian/rules` or
the dh-virtualenv build path uses it; the inner sdb wheel build
fetches its own copy via pyproject.toml's `[build-system].requires`.

Author: prakashsurya

debian/control

@@ -9,6 +9,7 @@ Build-Depends:
  python3-dev,
  python3-pip,
  python3-setuptools,
+ python3-setuptools-scm,
  python3-venv
 Standards-Version: 4.6.2
 Homepage: https://github.com/sdimitro/sdb