xnest: re-enable XKB support. #10015

XKB was disabled in 08928afb05, with the comment
"Disable XKB, as we can't yet use it". Seems like "yet" is over, running GNOME
and changing XKB settings seems to work in Xnest now.

X.Org Bug 10015 <https://bugs.freedesktop.org/show_bug.cgi?id=10015>

hw/xnest/Args.c

@@ -57,11 +57,6 @@ void ddxInitGlobals(void)
     extern Bool noCompositeExtension;
     noCompositeExtension = TRUE;
 #endif
-
-#ifdef XKB
-    extern Bool noXkbExtension;
-    noXkbExtension = TRUE;
-#endif
 }
 
 int