Fixes: warning: variable 'req' set but not,used

Fixes: warning: variable 'req' set but not used [-Wunused-but-set-variable]
       by marking req _X_UNUSED
	Solution was discussed on xorg-devel ML
       Peter Hutter, Alan Coopersmith
        Re: [PATCH libX11 3/5] fix: warning: pointer targets in passing argument 2 of '_XSend' differ in signedness [-Wpointer-sign]

Signed-off-by: harms <wharms@bfs.de>