netdev: Squash memory leak on module_init failure

In the case of module_init failing due to a module that comes after
netdev, the netdev module doesn't clean up netdev_list properly.

==6254== 24 bytes in 1 blocks are still reachable in loss record 1 of 1
==6254==    at 0x483777F: malloc (in
/usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==6254==    by 0x4675ED: l_malloc (util.c:61)
==6254==    by 0x46909D: l_queue_new (queue.c:63)
==6254==    by 0x406AE4: netdev_init (netdev.c:5038)
==6254==    by 0x44A7B3: iwd_modules_init (module.c:152)
==6254==    by 0x404713: nl80211_appeared (main.c:171)
==6254==    by 0x4713DE: process_unicast (genl.c:993)
==6254==    by 0x4713DE: received_data (genl.c:1101)
==6254==    by 0x46E00B: io_callback (io.c:118)
==6254==    by 0x46D20C: l_main_iterate (main.c:477)
==6254==    by 0x46D2DB: l_main_run (main.c:524)
==6254==    by 0x46D2DB: l_main_run (main.c:506)
==6254==    by 0x46D502: l_main_run_with_signal (main.c:656)
==6254==    by 0x403EDB: main (main.c:490)

src/netdev.c

@@ -5072,6 +5072,7 @@ static void netdev_exit(void)
 	l_genl_remove_unicast_watch(genl, unicast_watch);
 
 	watchlist_destroy(&netdev_watches);
+	l_queue_destroy(netdev_list, netdev_free);
 
 	l_genl_family_free(nl80211);
 	nl80211 = NULL;