Add inode compare while checking resolv file change

Fix a bug found on OpenWrt when IPv4/6 dual stack enabled:

The resolv file is located on tmpfs whose mtime resolution
is 1 second. If the resolv file is updated twice within one
second dnsmasq may can't notice the second update.

netifd updates the resolv file with method: write temp then move,
so adding an inode check fixes this bug.

src/dnsmasq.h

@@ -665,6 +665,7 @@ struct resolvc {
   struct resolvc *next;
   int is_default, logged;
   time_t mtime;
+  ino_t ino;
   char *name;
 #ifdef HAVE_INOTIFY
   int wd; /* inotify watch descriptor */