apple: use correct include folder s/Appkit/AppKit/

Fixes compilation on case sensitive macOS FS.

Signed-off-by: Yurii Kolesnykov <root@yurikoles.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
3 jobs for fix-macOS-cs in 1 minute and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Container
passed container #7800228

00:00:39

 
  Build
passed cmake #7800230

00:00:28

passed meson #7800229

00:00:37