Fix build on macOS 10.11

Setting an undefined property is a hard error, even though we check for
the property first. Using message notation instead reduces this to a
warning, and allows the build to continue on systems without
allowsAutomaticWindowTabbing.

Change-Id: I572d8900fdeca16c733d541084d64385891733c1
Reviewed-on: https://gerrit.libreoffice.org/63526
Tested-by: Jenkins
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>

vcl/osx/vclnsapp.mm

@@ -64,7 +64,7 @@
     (void)pNotification;
     if( [NSWindow respondsToSelector:@selector(allowsAutomaticWindowTabbing)] )
     {
-        NSWindow.allowsAutomaticWindowTabbing = NO;
+        [NSWindow setAllowsAutomaticWindowTabbing:NO];
     }
 }