In Linux 2.6.31 (or so) and earlier, the initramfs
decompression had its own compiled copy of the
decompression code that got thrown away after the
kernel had booted. It required that all functions were
marked with __init when built for initramfs decompression.

Nowadays zlib and LZO have a wrapper that requires that
the respective decompressor code has been been enabled (=y)
in the kernel config. Only the wrapper is marked with
__init. This patch helps doing the same with the XZ
decompressor.