po: set gettext domain in Makevars so we don't have to patch the generated Makefile.in.in

https://bugzilla.gnome.org/show_bug.cgi?id=705455

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 = $(GETTEXT_PACKAGE)
+DOMAIN = gst-plugins-good-1.0
 
 # These two variables depend on the location of this directory.
 subdir = po