xwayland: keep `xwl_present_timer_callback()` private

`xwl_present_timer_callback()` is initially marked a private and later
implemented as public.

Let's keep that private, shall we.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
1 job for trivial-fix in 1 minute and 51 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #30672
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00:01:51