gnome-online-accounts: Upcoming string change in stable branches

[Debarshi Ray <rishi is lostca se>]

Hello everybody,

Sadly, I need to introduce a new string to gnome-online-accounts.git
in the gnome-3-26 and gnome-3-24 branches due to this Kerberos bug:
  https://bugzilla.gnome.org/show_bug.cgi?id=789187

It adds this string as a label for a text entry for entering a
Kerberos principal:
  * "_Principal"

I promise to wait a few more weeks before rolling new tarballs from
gnome-3-26 and gnome-3-24.

Thanks,
Rishi