intl: Add missing libintl.h dependency from pluralx.$lo
plural(x).c depends on libintl.h if ENABLE_NLS, but the makefile
didn't have corresponding dependency for pluralx.c, because
d2d04ba forgot to add it. Fix by using $(PLURAL_OBJECT) for the
dependency rule too.
* Makefile.in ($(PLURAL_OBJECT)): Depend on libintl.h.
