libdw is missing from the platform

Context

This was reported originally in https://gitlab.gnome.org/GNOME/gnome-sdk-images/issues/13

Epiphany fails with the following error message:

libdw.so.1: cannot open shared object file

Description

$ flatpak run org.freedesktop.Platform//unstable
sh-4.4$ find /usr -name 'libdw*so'
sh-4.4$ exit
$ flatpak run org.freedesktop.Sdk//unstable
sh-4.4$ find /usr -name 'libdw*so'
/usr/lib/x86_64-linux-gnu/libdw.so
/usr/lib/x86_64-linux-gnu/libdw-0.172.so
sh-4.4$ exit

I think the problem is base/elfutils.bst is included in base.bst (which ends up in the Sdk) but not in base-platform.bst (which ends up in the Platform)

Acceptance Criteria

org.freedesktop.Platform//unstable should include libdw.