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.
issue