man/sd_journal_next: fix argument in example

The example does not compile, it fails with:

error: passing argument 3 of ‘sd_journal_get_data’ from incompatible
pointer type

Cast to (const void **) to avoid this.

(cherry picked from commit 8959ae0d4c)

man/sd_journal_next.xml

@@ -183,7 +183,7 @@ int main(int argc, char *argv[]) {
                 const char *d;
                 size_t l;
 
-                r = sd_journal_get_data(j, "MESSAGE", &d, &l);
+                r = sd_journal_get_data(j, "MESSAGE", (const void **)&d, &l);
                 if (r < 0) {
                         fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
                         continue;