src: fix --without-inspector build

`use_inspector` is not available if `HAVE_INSPECTOR` is false.
Before this commit, one usage of it would show up as an undeclared
identifier (introduced in a766ebf).

PR-URL: nodejs#7258
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node.cc

@@ -3359,11 +3359,15 @@ static bool ParseDebugOpt(const char* arg) {
       PrintHelp();
       exit(12);
     }
+#if HAVE_INSPECTOR
     if (use_inspector) {
       inspector_port = port_int;
     } else {
+#endif
       debug_port = port_int;
+#if HAVE_INSPECTOR
     }
+#endif
   }
 
   return true;