XZ_EXTERN was used to make internal functions static in the preboot code. 
However, in other decompressors this hasn't been done.  On x86-64, this
makes no difference to the kernel image size.

Omit XZ_EXTERN and let some of the internal functions be extern in the
preboot code.  Omitting XZ_EXTERN from include/linux/xz.h fixes warnings
in "make htmldocs" and makes the intradocument links to xz_dec functions
work in Documentation/staging/xz.rst.  The alternative would have been to
add "XZ_EXTERN" to c_id_attributes in Documentation/conf.py but omitting
XZ_EXTERN seemed cleaner.

Link: https://lore.kernel.org/lkml/20240723205437.3c0664b0@kaneli/
Link: https://lkml.kernel.org/r/20240724110544.16430-1-lasse.collin@tukaani.orgSigned-off-by: Lasse Collin <lasse.collin@tukaani.org>
Tested-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sam James <sam@gentoo.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Jubin Zhong <zhongjubin@huawei.com>
Cc: Jules Maselbas <jmaselbas@zdiv.net>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Rui Li <me@lirui.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>