journalctl: add remote log dir to search path when --merge is passed (#4970)

The journalctl man page says: "-m, --merge Show entries interleaved from all
available journals, including remote ones.", but current version of journalctl
doesn't live up to this promise. This patch simply adds
"/var/log/journal/remote" to search path if --merge flag is used.

Should fix issue #3618

src/journal/sd-journal.c

@@ -1661,6 +1661,9 @@ static int add_search_paths(sd_journal *j) {
         NULSTR_FOREACH(p, search_paths)
                 (void) add_root_directory(j, p, true);
 
+        if (!(j->flags & SD_JOURNAL_LOCAL_ONLY))
+                (void) add_root_directory(j, "/var/log/journal/remote", true);
+
         return 0;
 }