build: Remove xgettext _g keyword for the dpkg domain

The dpkg domain consists only of C sources, so there's no need anymore
for the Perl specific _g keyword.

po/Makevars

@@ -8,7 +8,7 @@ subdir = po
 top_builddir = ..
 
 # These options get passed to xgettext.
-XGETTEXT_OPTIONS = --keyword=_ --keyword=N_ --keyword=_g --from-code=UTF-8
+XGETTEXT_OPTIONS = --keyword=_ --keyword=N_ --from-code=UTF-8
 
 MSGMERGE_OPTIONS = --previous