ui/gtk: Fix build with older versions of gtk

GDK_KEY_Delete is only defined with gtk version 2.22 and newer,
on older versions this key was called GDK_Delete instead.
Since this is the case for all GDK_KEY_* defines, change the
already existing preprocessor check there to test for version 2.22,
so we know that we can remove this code block in case we require
that version as a minimum one day.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1478081328-25515-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
