hw/xwin: Fix compilation with -Werror=return-type

winshadddnl.c: In function ‘winRedrawScreenShadowDDNL’:
winshadddnl.c:991:9: error: ‘return’ with no value, in function returning non-void [-Werror=return-type]

Just wrong in 1c34e774

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

hw/xwin/winshadddnl.c

@@ -988,7 +988,7 @@ winRedrawScreenShadowDDNL(ScreenPtr pScreen)
 
     /* Return immediately if we didn't get needed surfaces */
     if (!pScreenPriv->pddsPrimary4 || !pScreenPriv->pddsShadow4)
-        return;
+        return FALSE;
 
     /* Get the origin of the window in the screen coords */
     ptOrigin.x = pScreenInfo->dwXOffset;