[Minor] use quilt for debian/source/format

* Match the build scripts and main Debian packaging.
* This allows for a separate debian/ directory in debian.tar.gz, see
  https://www.debian.org/doc/manuals/maint-guide/dother.en.html#sourcef

debian/source/format

@@ -1 +1 @@
-3.0 (native)
+3.0 (quilt)