gtkbuilder: Install gettext ITS files

Recent gettext has a feature to allow consumer projects to supply their
own string extraction rules for XML files, in ITS format.

Gettext still ships the rule for *.ui, but it would be better
maintained in the upstream project.

https://bugzilla.gnome.org/show_bug.cgi?id=760202

gtk/Makefile.am

@@ -1114,6 +1114,11 @@ pkgdata_DATA = gtkbuilder.rng
 
 EXTRA_DIST += gtkbuilder.rnc gtkbuilder.rng
 
+itsdir = $(datadir)/gettext/its
+its_DATA = gtkbuilder.loc gtkbuilder.its
+
+EXTRA_DIST += $(its_DATA)
+
 templates =				\
 	$(inspector_templates)		\
 	ui/gtkapplication-quartz.ui	\