[gtk/gtk-3-24] Silence incompatible type warning.

[John Ralls <jralls src gnome org>]

commit 512da667616af4c3d29f38bf658c8bbe8ad63973
Author: John Ralls <jralls ceridwen us>
Date:   Tue Feb 19 11:07:48 2019 -0800

    Silence incompatible type warning.

 gdk/quartz/GdkQuartzNSWindow.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gdk/quartz/GdkQuartzNSWindow.c b/gdk/quartz/GdkQuartzNSWindow.c
index 92fb8079bd..8d1ba4af2b 100644
--- a/gdk/quartz/GdkQuartzNSWindow.c
+++ b/gdk/quartz/GdkQuartzNSWindow.c
@@ -278,7 +278,7 @@
                              screen:screen];
 
   [self setAcceptsMouseMovedEvents:YES];
-  [self setDelegate:self];
+  [self setDelegate:(id<NSWindowDelegate>)self];
   [self setReleasedWhenClosed:YES];
 
   return self;