[Debian] Need to explicitly list g++ as a package dependency

[grynspan]

The post-install script for Debian lists a bunch of packages that may be needed, but on Debian (at least), g++ is a separate package from gcc and needs to be explicitly listed.

[etcwilde]

Hello, can you provide a bit more information about what is not working, including swiftly version, toolchain version, and something about what you're trying to build. Swift itself uses clang/clang++.
Thank you.