CDRIVER-4673 fix include of bson-dsl.h (#1388)

* move bson-dsl.h to src/common

bson-dsl.h is not installed, but shared between libmongoc and libbson.
Intended to resolve CDRIVER-4673.

* format bson-dsl.h

* add bson-dsl.h to dist tarball

* remove bson-dsl.h from old dist list

* add regression test