Translations: Add src/xz/sandbox.c to POTFILES.in

There are no strings to translate in that file now, but it's good to
list it anyway in case translatable strings are added in the future.

Fixes: 374868d ("xz: Move sandboxing code to sandbox.c and improve Landlock sandbox.")

Author: Larhzu

po/POTFILES.in

@@ -10,6 +10,7 @@ src/xz/main.c
 src/xz/message.c
 src/xz/mytime.c
 src/xz/options.c
+src/xz/sandbox.c
 src/xz/signals.c
 src/xz/suffix.c
 src/xz/util.c