xfig: Fix build failure now visible due to #7524

The first 'find' command in postUnpack that attempts to add +x
permissions fails, because the +x permission is needed by the find
itself, and with the xargs method it's added too late.

pkgs/applications/graphics/xfig/builder.sh

···
 # We need chmod +wx on dirs, not just chmod +w
 dontMakeSourcesWritable=1
 postUnpack() {
-	find . -type d | xargs -n1 chmod +x
-	find . -type d | xargs -n1 chmod +x
+    find . -type d -exec chmod +x '{}' \;
 }
 
 preBuild() {