meson: Redefine 'datadir' instead of 'prefix'.
meson: Redefine 'datadir' instead of 'prefix'.

bash-completion.pc uses $datadir to calculate $completionsdir, so
redefining $prefix doesn't help.

With this change, waffle will respect --prefix when installing its
bash-completion file, instead of using bash-completion's prefix.

Acked-by: Eric Engestrom <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@engestrom.ch>

waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle