decompressor: provide missing prototypes

The entry points for the decompressor don't always have a prototype
included in the .c file:

lib/decompress_inflate.c:42:17: error: no previous prototype for '__gunzip' [-Werror=missing-prototypes]
lib/decompress_unxz.c:251:17: error: no previous prototype for 'unxz' [-Werror=missing-prototypes]
lib/decompress_unzstd.c:331:17: error: no previous prototype for 'unzstd' [-Werror=missing-prototypes]

Include the correct headers for unxz and unzstd, and mark the inflate
function above as unconditionally 'static' to avoid these warnings.

Link: https://lkml.kernel.org/r/20230517131936.936840-1-arnd@kernel.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>