ximcp/imRm.c: allow XNSpotLocation with OnTheSpot

The XNSpotLocation getting/setting wasn't allowed when preedit callbacks
were used, the original motivation about this limitation was that xim
servers weren't handling it position with preedit callbacks, however in
the current state I'm not sure if there's a single xim server that could
break from it, and it's more desired for the end user.

Moreover libxcb does allow doing so, so if the client is using libxcb
the XNSpotLocation could be set, unlike with libX11. So for the end user
it looks like libX11 clients are buggy compared to libxcb ones, since
their IME popup window is being in the wrong location with the libX11.

The change was already brought in the past in [1], but it seems like it
got lost.

[1] - https://bugs.freedesktop.org/show_bug.cgi?id=1580.

Signed-off-by: Kirill Chibisov <contact@kchibisov.com>
6 jobs for on-spot-pos in 6 minutes and 5 seconds (queued for 3 seconds)