build: Fix gettext domain after recent changes

Commit 1b11db36 that switched the build
system to use upstream gettext intead glib-gettext accidentally changed
the gettext domain from atk10 to atk; this commit changes it back.

https://bugzilla.gnome.org/show_bug.cgi?id=763212

po/Makevars

 # Makefile variables for PO directory in any package using GNU gettext.
 
 # Usually the message domain is the same as the package name.
-DOMAIN = $(PACKAGE)
+DOMAIN = atk10
 
 # These two variables depend on the location of this directory.
 subdir = po