Fix visibiity of gdkselection-quartz.c exported gtk-only functions.

Also ensure that gdkquartz-gtk-only.h is included in distribution
tarballs.

Failing to include gdkquartz-gtk-only.h in gdkselection-quartz.c
caused the compiler to not set the extern storage class on the
functions, in turn causing them to be not exported by libgdk-3.0.dylib.

Fixes https://gitlab.gnome.org/GNOME/gtk/issues/32 again.

gdk/quartz/Makefile.am

@@ -60,6 +60,7 @@ libgdkinclude_HEADERS = 	\
 	gdkquartz.h
 
 libgdkquartzinclude_HEADERS =		\
+	gdkquartz-gtk-only.h            \
 	gdkquartzcursor.h		\
 	gdkquartzdevice-core.h		\
 	gdkquartzdevicemanager-core.h	\

gdk/quartz/gdkselection-quartz.c

@@ -24,6 +24,7 @@
 #include "gdkproperty.h"
 #include "gdkquartz.h"
 #include "gdkinternal-quartz.h"
+#include "gdkquartz-gtk-only.h"
 
 gboolean
 _gdk_quartz_display_set_selection_owner (GdkDisplay *display,