fix warning:
> ../os/rpcauth.c:159:16: warning: result of comparison of constant -1 with expression of type 'XID' (aka 'unsigned int') is always false [-Wtautological-constant-out-of-range-compare]
> if (rpc_id == ~0L)
> ~~~~~~ ^ ~~~
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1431>