Build failure with trunk GCC 15 in fs/btrfs/print-tree.c (-Wunterminated-string-initialization)

[Sam James <sam@gentoo.org>]

GCC 15 introduces a new warning -Wunterminated-string-initialization
which causes, with the kernel's -Werror=..., the following:
```
/var/tmp/portage/sys-kernel/gentoo-kernel-6.6.41/work/linux-6.6/fs/btrfs/print-tree.c:29:49: error: initializer-string for array of ‘char’ is too long [-Werror=unterminated-string-initialization]
   29 |         { BTRFS_BLOCK_GROUP_TREE_OBJECTID,      "BLOCK_GROUP_TREE"      },
      |
      ^~~~~~~~~~~~~~~~~~
```

It was introduced in https://gcc.gnu.org/PR115185. I don't have time
today to check the case to see what the best fix is, but CCing Alex who
wrote the warning implementation in case he has a chance.

thanks,
sam