sd-journal: do not require path to be absolute

Seems natural to be able to specify relative directory,
e.g. with journalctl -D. And even if, this should be checked
in front-end code, not in the library.

src/journal/sd-journal.c

@@ -1596,7 +1596,7 @@ _public_ int sd_journal_open_directory(sd_journal **ret, const char *path, int f
         if (!ret)
                 return -EINVAL;
 
-        if (!path || !path_is_absolute(path))
+        if (!path)
                 return -EINVAL;
 
         if (flags != 0)