wibox.widget.systray: fix error while drawing widget

> bad argument #2 to 'systray' (number has no integer representation)

Closes https://github.com/awesomeWM/awesome/pull/247

lib/wibox/widget/systray.lua.in

@@ -41,7 +41,8 @@ function systray:draw(wibox, cr, width, height)
     else
         base = in_dir / num_entries
     end
-    capi.awesome.systray(wibox.drawin, x, y, base, is_rotated, bg, reverse, spacing)
+    capi.awesome.systray(wibox.drawin, math.ceil(x), math.ceil(y),
+                         base, is_rotated, bg, reverse, spacing)
 end
 
 function systray:fit(width, height)