resolved: use macro for private resolve.conf

Use the macro from header file to have a single definition of the path.

src/resolve/resolved-resolv-conf.c

@@ -60,7 +60,7 @@ int manager_read_resolv_conf(Manager *m) {
                 return 0;
 
         /* Is it symlinked to our own file? */
-        if (stat("/run/systemd/resolve/resolv.conf", &own) >= 0 &&
+        if (stat(PRIVATE_RESOLV_CONF, &own) >= 0 &&
             st.st_dev == own.st_dev &&
             st.st_ino == own.st_ino)
                 return 0;