src: install empathy-main-window-menubar.ui

src: install empathy-main-window-menubar.ui

empathy-main-window-menubar.ui was added in this commit:

   1ad15d157971b6550cb11787a03463e891d29364 is the first bad commit
   commit 1ad15d157971b6550cb11787a03463e891d29364
   Author: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
   Date:   Tue Aug 23 14:08:18 2011 +0200

      Move main window's UI manager to its own UI file

      Glade doesn't support GtkUIManager so it was impossible to edit
      the main window at all because of that.

But was not marked to be installed.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=657271