patch 8.0.1606: singular/plural variants not translated

Problem:    Singular/plural variants not translated.
Solution:   Add NGETTEXT argument to xgettext. (Sergey Alyoshin)
diff --git a/src/po/Makefile b/src/po/Makefile
index 8c4aaf3..fc78c43 100644
--- a/src/po/Makefile
+++ b/src/po/Makefile
@@ -283,7 +283,7 @@
 
 $(PACKAGE).pot: ../*.c ../if_perl.xs ../GvimExt/gvimext.cpp ../globals.h ../if_py_both.h
 	cd ..; $(XGETTEXT) --default-domain=$(PACKAGE) \
-		--add-comments --keyword=_ --keyword=N_ \
+		--add-comments --keyword=_ --keyword=N_ --keyword=NGETTEXT:1,2 \
 		*.c if_perl.xs GvimExt/gvimext.cpp globals.h if_py_both.h
 	mv -f ../$(PACKAGE).po $(PACKAGE).pot