Fix hidden dependency issue with btrfs and crc32c

Quoting Markus Wanner in #748805:

| Since Linux 3.14 (or 0b947aff1599afbbd2ec07ada87b05af0f94cf10, to be
| precise), the btrfs module no longer depends on libcrc32c, but only on
| crc32c. However, this is one of the "hidden" dependencies, so
| modules.dep doesn't list it. If mkinitramfs doesn't happen to include
| crc32c for some other reason, an initrd without that module is
| generated, even if btrfs needs it to boot. For me, this led to the same
| error upon boot, as others have posted, before:
|
| modprobe: can't load module btrfs (kernel/fs/btrfs/btrfs.ko): unknown
| symbol in module, or unknown parametr
|
| (Without any further hints in dmegs, BTW)
|
| The attached patch adds an entry to the list of hidden dependencies to
| /usr/share/initramfs-tools/hook-functions to fix this issue. This also
| renders the work-around proposed by Tristan unnecessary.

Thanks: Markus Wanner <markus@bluegap.ch> for the analysis and patch
Closes: #748805