Build error with clang 4.0

[Paul Menzel]

Dear GRUB folks,


Using Clang 4.0 the build fails with the error below.

```
$ sudo apt install clang-4.0 # Debian Sid/unstable
$ clang-4.0 --version
clang version 4.0.0-svn286225-1 (trunk)
Target: i686-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
$ git log --oneline -1
ce95549cc efi: properly terminate filepath with NULL in chainloader
$ ./autoconf.sh
$ CC=clang-4.0 ../grub/configure --with-platform=coreboot --enable-boot-time
$ make -j
[…]
../grub/grub-core/fs/hfs.c:699:10: error: taking address of packed member 
'catalog_recs' of class or structure 'grub_hfs_sblock' may result in an 
unaligned pointer
      value [-Werror,-Waddress-of-packed-member]
                                        ? (&data->sblock.catalog_recs)
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~
../grub/grub-core/fs/hfs.c:700:10: error: taking address of packed member 
'extent_recs' of class or structure 'grub_hfs_sblock' may result in an 
unaligned pointer
      value [-Werror,-Waddress-of-packed-member]
                                        : (&data->sblock.extent_recs));
                                            ^~~~~~~~~~~~~~~~~~~~~~~~
[…]
```

The build succeeds using gcc (Debian 6.2.1-7) 6.2.1 20161215, so I am
unsure if this is a Clang issue.


Kind regards,

Paul

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