control: build-depend on gnome-control-center-data

While these are not used during the normal build of gnome-shell, extracting strings from data/50-gnome-shell-system.xml depends on gettext rules which are shipped by the gnome-control-center-data package. Without these, ninja gnome-shell-pot warns:

xgettext: warning: file 'data/50-gnome-shell-system.xml' extension 'xml' is unknown; will try C

and the relevant strings are not extracted.

I noticed this because, in Endless, we add new translatable strings and so need to regenerate the .pot file. It would be ideal if apt build-dep for a package would install all files needed to both build the package, and also generate its .pot file, since this would not need to be special-cased.

I'm open to persuasion that:

• we should carry this change downstream since it's not needed in normal use
• these files should instead be shipped in gnome-control-center-dev since (I claim) they're never needed during normal use of g-c-c