random-seed: terminate the option array

So we don't crash on invalid options:

$ build/systemd-random-seed --foo
Segmentation fault (core dumped)

src/random-seed/random-seed.c

@@ -348,6 +348,7 @@ static int parse_argv(int argc, char *argv[]) {
         static const struct option options[] = {
                 { "help",    no_argument, NULL, 'h'         },
                 { "version", no_argument, NULL, ARG_VERSION },
+                {}
         };
 
         int c;