x11/gnome-shell: list bash-completion in BUILD_DEPENDS

This is to isolate the outcome of the build of gnome-shell
from options in other ports, namely p11-kit's BASH and
gnome-control-center's CHEESE options.

We may want to consider adding bash-completion to RUN_DEPENDS,
too, this is up for further discussion.

PR: 250269 (related)
Reported by: cmt@
Approved by: portmgr@ (blanket, fix dependencies)