xkb: Make the RT_XKBCLIENT resource private

Currently, the resource in only available to the xkb.c source file.

In preparation for the next commit, to be able to free the resources
from XkbRemoveResourceClient(), make that variable private instead.

This is related to:

CVE-2025-62230, ZDI-CAN-27545

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

Signed-off-by: Olivier Fourdan <[email protected]>
Reviewed-by: Michel Dänzer <[email protected]>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2086>

Author: ofourdan

xkb/xkb.c

@@ -53,7 +53,7 @@ int XkbKeyboardErrorCode;
 CARD32 xkbDebugFlags = 0;
 static CARD32 xkbDebugCtrls = 0;
 
-static RESTYPE RT_XKBCLIENT;
+RESTYPE RT_XKBCLIENT = 0;
 
 /***====================================================================***/
 

xkb/xkbsrv_priv.h

@@ -98,6 +98,8 @@
         device->public.realInputProc = oldprocs->realInputProc; \
         device->unwrapProc = oldprocs->unwrapProc;
 
+extern RESTYPE RT_XKBCLIENT;
+
 void xkbUnwrapProc(DeviceIntPtr, DeviceHandleProc, void *);
 
 void XkbForceUpdateDeviceLEDs(DeviceIntPtr keybd);