xkb: free XkbRulesUsed and XkbRulesDflt on extension cleanup

==2547== 1 bytes in 1 blocks are still reachable in loss record 1 of 111
==2547==    at 0x4C2A4CD: malloc (vg_replace_malloc.c:236)
==2547==    by 0x64D1551: strdup (strdup.c:43)
==2547==    by 0x4802FB: Xstrdup (utils.c:1113)
==2547==    by 0x585B6C: XkbSetRulesUsed (xkbInit.c:219)
==2547==    by 0x58700F: InitKeyboardDeviceStruct (xkbInit.c:595)
==2547==    by 0x419FA3: vfbKeybdProc (InitInput.c:74)
==2547==    by 0x425A3D: ActivateDevice (devices.c:540)
==2547==    by 0x425F65: InitAndStartDevices (devices.c:713)
==2547==    by 0x5ACA57: main (main.c:259)

and a few more of the above.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>

include/xkbsrv.h

@@ -751,6 +751,9 @@ extern _X_EXPORT void XkbSetRulesDflts(XkbRMLVOSet *    /* rmlvo */
 extern _X_EXPORT void XkbDeleteRulesDflts(void
     );
 
+extern _X_EXPORT void XkbDeleteRulesUsed(void
+    );
+
 extern _X_EXPORT int SProcXkbDispatch(ClientPtr /* client */
     );