macos: Fix gdk_quartz_drag_context_get_dragging_info_libgtk_only symbol export

The header got included without config.h being included first which resulted in the
wrong _GDK_EXTERN macro being used. As a result some symbols weren't exported
and starting a DnD action would crash in the linker.

This patch adds config.h includes in all places where clang complained about
_GDK_EXTERN redefinitions.

See #32 for more info.

gdk/quartz/GdkQuartzNSWindow.c

@@ -16,6 +16,7 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "config.h"
 #import "GdkQuartzNSWindow.h"
 #include "gdkquartzwindow.h"
 #include "gdkdnd-quartz.h"

gdk/quartz/GdkQuartzView.c

@@ -17,6 +17,7 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "config.h"
 #import "GdkQuartzView.h"
 #include "gdkquartzwindow.h"
 #include "gdkprivate-quartz.h"

gdk/quartz/gdkdnd-quartz.c

@@ -16,6 +16,7 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "config.h"
 #include "gdkdnd.h"
 #include "gdkquartzdnd.h"
 #include "gdkprivate-quartz.h"