block/qcow2-bitmap: Add missing cast to silent GCC error

Commit d1258dd0 ("qcow2: autoloading dirty bitmaps") added the
set_readonly_helper() GFunc handler, correctly casting the gpointer
user_data in both the g_slist_foreach() caller and the handler.
Few commits later (commit 1b6b0562), the handler is reused in
qcow2_reopen_bitmaps_rw() but missing the gpointer cast, resulting
in the following error when using Homebrew GCC 12.2.0:

  [2/658] Compiling C object libblock.fa.p/block_qcow2-bitmap.c.o
  ../../block/qcow2-bitmap.c: In function 'qcow2_reopen_bitmaps_rw':
  ../../block/qcow2-bitmap.c:1211:60: error: incompatible type for argument 3 of 'g_slist_foreach'
   1211 |     g_slist_foreach(ro_dirty_bitmaps, set_readonly_helper, false);
        |                                                            ^~~~~
        |                                                            |
        |                                                            _Bool
  In file included from /opt/homebrew/Cellar/glib/2.72.3_1/include/glib-2.0/glib/gmain.h:26,
                   from /opt/homebrew/Cellar/glib/2.72.3_1/include/glib-2.0/glib/giochannel.h:33,
                   from /opt/homebrew/Cellar/glib/2.72.3_1/include/glib-2.0/glib.h:54,
                   from /Users/philmd/source/qemu/include/glib-compat.h:32,
                   from /Users/philmd/source/qemu/include/qemu/osdep.h:144,
                   from ../../block/qcow2-bitmap.c:28:
  /opt/homebrew/Cellar/glib/2.72.3_1/include/glib-2.0/glib/gslist.h:127:61: note: expected 'gpointer' {aka 'void *'} but argument is of type '_Bool'
    127 |                                           gpointer          user_data);
        |                                           ~~~~~~~~~~~~~~~~~~^~~~~~~~~
  At top level:
  FAILED: libblock.fa.p/block_qcow2-bitmap.c.o

Fix by adding the missing gpointer cast.

Fixes: 1b6b0562 ("qcow2: support .bdrv_reopen_bitmaps_rw")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20220919182755.51967-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

block/qcow2-bitmap.c

@@ -1208,7 +1208,7 @@ int qcow2_reopen_bitmaps_rw(BlockDriverState *bs, Error **errp)
         }
     }
 
-    g_slist_foreach(ro_dirty_bitmaps, set_readonly_helper, false);
+    g_slist_foreach(ro_dirty_bitmaps, set_readonly_helper, (gpointer)false);
     ret = 0;
 
 out: