meson: Small tweak to use Diet as a subproject

[ximion]

This makes using Diet as a Meson suproject a lot easier, because it allows using a shortcut in other Meson projects (just pulling in the diet_dep internal dependency, instead of redefining it in every project)

[s-ludwig]

Shall I also tag a new version so that this works for vibe.d?

[ximion]

If you want to... That depends on how you use the wrap file in Vibe (see a small explanation on the other PR).
For the Debian packaging, having a new version would be nice, but also not essential (since this patch only affects the case when some other project is embedding Diet, while on Linux distributions we will always want to use a system copy).